package storage

import (
	"bytes"
	"container/heap"
	"errors"
	"fmt"
	"io"
	"path/filepath"
	"sort"
	"sync"
	"sync/atomic"
	"time"
	"unsafe"

	"github.com/VictoriaMetrics/VictoriaMetrics/lib/bytesutil"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/encoding"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fasttime"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fs"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/memory"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/mergeset"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/uint64set"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/workingsetcache"
	"github.com/VictoriaMetrics/fastcache"
	xxhash "github.com/cespare/xxhash/v2"
)

const (
	// Prefix for MetricName->TSID entries.
	nsPrefixMetricNameToTSID = 0

	// Prefix for Tag->MetricID entries.
	nsPrefixTagToMetricIDs = 1

	// Prefix for MetricID->TSID entries.
	nsPrefixMetricIDToTSID = 2

	// Prefix for MetricID->MetricName entries.
	nsPrefixMetricIDToMetricName = 3

	// Prefix for deleted MetricID entries.
	nsPrefixDeletedMetricID = 4

	// Prefix for Date->MetricID entries.
	nsPrefixDateToMetricID = 5

	// Prefix for (Date,Tag)->MetricID entries.
	nsPrefixDateTagToMetricIDs = 6
)

func shouldCacheBlock(item []byte) bool {
	if len(item) == 0 {
		return true
	}
	// Do not cache items starting from
	switch item[0] {
	case nsPrefixTagToMetricIDs, nsPrefixDateTagToMetricIDs:
		// Do not cache blocks with tag->metricIDs and (date,tag)->metricIDs items, since:
		// - these blocks are scanned sequentially, so the overhead
		//   on their unmarshaling is amortized by the sequential scan.
		// - these blocks can occupy high amounts of RAM in cache
		//   and evict other frequently accessed blocks.
		return false
	case nsPrefixDeletedMetricID:
		// Do not cache blocks with deleted metricIDs,
		// since these metricIDs are loaded only once during app start.
		return false
	default:
		return true
	}
}

// indexDB represents an index db.
type indexDB struct {
	// Atomic counters must go at the top of the structure in order to properly align by 8 bytes on 32-bit archs.
	// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/212 .

	refCount uint64

	// The counter for newly created time series. It can be used for determining time series churn rate.
	newTimeseriesCreated uint64

	// The number of missing MetricID -> TSID entries.
	// High rate for this value means corrupted indexDB.
	missingTSIDsForMetricID uint64

	// The number of calls to search for metric ids for recent hours.
	recentHourMetricIDsSearchCalls uint64

	// The number of cache hits during search for metric ids in recent hours.
	recentHourMetricIDsSearchHits uint64

	// The number of searches for metric ids by days.
	dateMetricIDsSearchCalls uint64

	// The number of successful searches for metric ids by days.
	dateMetricIDsSearchHits uint64

	// The number of calls for date range searches.
	dateRangeSearchCalls uint64

	// The number of hits for date range searches.
	dateRangeSearchHits uint64

	// missingMetricNamesForMetricID is a counter of missing MetricID -> MetricName entries.
	// High rate may mean corrupted indexDB due to unclean shutdown.
	// The db must be automatically recovered after that.
	missingMetricNamesForMetricID uint64

	mustDrop uint64

	// Start date fully covered by per-day inverted index.
	startDateForPerDayInvertedIndex uint64

	name string
	tb   *mergeset.Table

	extDB     *indexDB
	extDBLock sync.Mutex

	// Cache for fast TagFilters -> TSIDs lookup.
	tagCache *workingsetcache.Cache

	// Cache for fast MetricID -> TSID lookup.
	metricIDCache *workingsetcache.Cache

	// Cache for fast MetricID -> MetricName lookup.
	metricNameCache *workingsetcache.Cache

	// Cache for fast MetricName -> TSID lookups.
	tsidCache *workingsetcache.Cache

	// Cache for useless TagFilters entries, which have no tag filters
	// matching low number of metrics.
	uselessTagFiltersCache *workingsetcache.Cache

	// Cache for (date, tagFilter) -> metricIDsLen, which is used for reducing
	// the amount of work when matching a set of filters.
	metricIDsPerDateTagFilterCache *workingsetcache.Cache

	indexSearchPool sync.Pool

	// An inmemory set of deleted metricIDs.
	//
	// The set holds deleted metricIDs for the current db and for the extDB.
	//
	// It is safe to keep the set in memory even for big number of deleted
	// metricIDs, since it usually requires 1 bit per deleted metricID.
	deletedMetricIDs           atomic.Value
	deletedMetricIDsUpdateLock sync.Mutex

	// Global lists of metric ids for the current and the previous hours.
	// They are used for fast lookups on small time ranges covering
	// up to two last hours.
	currHourMetricIDs *atomic.Value
	prevHourMetricIDs *atomic.Value
}

// openIndexDB opens index db from the given path with the given caches.
func openIndexDB(path string, metricIDCache, metricNameCache, tsidCache *workingsetcache.Cache, currHourMetricIDs, prevHourMetricIDs *atomic.Value) (*indexDB, error) {
	if metricIDCache == nil {
		logger.Panicf("BUG: metricIDCache must be non-nil")
	}
	if metricNameCache == nil {
		logger.Panicf("BUG: metricNameCache must be non-nil")
	}
	if tsidCache == nil {
		logger.Panicf("BUG: tsidCache must be nin-nil")
	}
	if currHourMetricIDs == nil {
		logger.Panicf("BUG: currHourMetricIDs must be non-nil")
	}
	if prevHourMetricIDs == nil {
		logger.Panicf("BUG: prevHourMetricIDs must be non-nil")
	}

	tb, err := mergeset.OpenTable(path, invalidateTagCache, mergeTagToMetricIDsRows)
	if err != nil {
		return nil, fmt.Errorf("cannot open indexDB %q: %w", path, err)
	}

	name := filepath.Base(path)

	// Do not persist tagCache in files, since it is very volatile.
	mem := memory.Allowed()

	db := &indexDB{
		refCount: 1,
		tb:       tb,
		name:     name,

		tagCache:                       workingsetcache.New(mem/32, time.Hour),
		metricIDCache:                  metricIDCache,
		metricNameCache:                metricNameCache,
		tsidCache:                      tsidCache,
		uselessTagFiltersCache:         workingsetcache.New(mem/128, time.Hour),
		metricIDsPerDateTagFilterCache: workingsetcache.New(mem/128, time.Hour),

		currHourMetricIDs: currHourMetricIDs,
		prevHourMetricIDs: prevHourMetricIDs,
	}

	is := db.getIndexSearch()
	dmis, err := is.loadDeletedMetricIDs()
	db.putIndexSearch(is)
	if err != nil {
		return nil, fmt.Errorf("cannot load deleted metricIDs: %w", err)
	}
	db.setDeletedMetricIDs(dmis)

	is = db.getIndexSearch()
	date, err := is.getStartDateForPerDayInvertedIndex()
	db.putIndexSearch(is)
	if err != nil {
		return nil, fmt.Errorf("cannot obtain start date for per-day inverted index: %w", err)
	}
	db.startDateForPerDayInvertedIndex = date

	return db, nil
}

// IndexDBMetrics contains essential metrics for indexDB.
type IndexDBMetrics struct {
	TagCacheSize      uint64
	TagCacheSizeBytes uint64
	TagCacheRequests  uint64
	TagCacheMisses    uint64

	UselessTagFiltersCacheSize      uint64
	UselessTagFiltersCacheSizeBytes uint64
	UselessTagFiltersCacheRequests  uint64
	UselessTagFiltersCacheMisses    uint64

	DeletedMetricsCount uint64

	IndexDBRefCount uint64

	NewTimeseriesCreated    uint64
	MissingTSIDsForMetricID uint64

	RecentHourMetricIDsSearchCalls uint64
	RecentHourMetricIDsSearchHits  uint64
	DateMetricIDsSearchCalls       uint64
	DateMetricIDsSearchHits        uint64

	DateRangeSearchCalls uint64
	DateRangeSearchHits  uint64

	MissingMetricNamesForMetricID uint64

	IndexBlocksWithMetricIDsProcessed      uint64
	IndexBlocksWithMetricIDsIncorrectOrder uint64

	mergeset.TableMetrics
}

func (db *indexDB) scheduleToDrop() {
	atomic.AddUint64(&db.mustDrop, 1)
}

// UpdateMetrics updates m with metrics from the db.
func (db *indexDB) UpdateMetrics(m *IndexDBMetrics) {
	var cs fastcache.Stats

	cs.Reset()
	db.tagCache.UpdateStats(&cs)
	m.TagCacheSize += cs.EntriesCount
	m.TagCacheSizeBytes += cs.BytesSize
	m.TagCacheRequests += cs.GetBigCalls
	m.TagCacheMisses += cs.Misses

	cs.Reset()
	db.uselessTagFiltersCache.UpdateStats(&cs)
	m.UselessTagFiltersCacheSize += cs.EntriesCount
	m.UselessTagFiltersCacheSizeBytes += cs.BytesSize
	m.UselessTagFiltersCacheRequests += cs.GetCalls
	m.UselessTagFiltersCacheMisses += cs.Misses

	m.DeletedMetricsCount += uint64(db.getDeletedMetricIDs().Len())

	m.IndexDBRefCount += atomic.LoadUint64(&db.refCount)
	m.NewTimeseriesCreated += atomic.LoadUint64(&db.newTimeseriesCreated)
	m.MissingTSIDsForMetricID += atomic.LoadUint64(&db.missingTSIDsForMetricID)
	m.RecentHourMetricIDsSearchCalls += atomic.LoadUint64(&db.recentHourMetricIDsSearchCalls)
	m.RecentHourMetricIDsSearchHits += atomic.LoadUint64(&db.recentHourMetricIDsSearchHits)
	m.DateMetricIDsSearchCalls += atomic.LoadUint64(&db.dateMetricIDsSearchCalls)
	m.DateMetricIDsSearchHits += atomic.LoadUint64(&db.dateMetricIDsSearchHits)

	m.DateRangeSearchCalls += atomic.LoadUint64(&db.dateRangeSearchCalls)
	m.DateRangeSearchHits += atomic.LoadUint64(&db.dateRangeSearchHits)

	m.MissingMetricNamesForMetricID += atomic.LoadUint64(&db.missingMetricNamesForMetricID)

	m.IndexBlocksWithMetricIDsProcessed = atomic.LoadUint64(&indexBlocksWithMetricIDsProcessed)
	m.IndexBlocksWithMetricIDsIncorrectOrder = atomic.LoadUint64(&indexBlocksWithMetricIDsIncorrectOrder)

	db.tb.UpdateMetrics(&m.TableMetrics)
	db.doExtDB(func(extDB *indexDB) {
		extDB.tb.UpdateMetrics(&m.TableMetrics)
		m.IndexDBRefCount += atomic.LoadUint64(&extDB.refCount)
	})
}

func (db *indexDB) doExtDB(f func(extDB *indexDB)) bool {
	db.extDBLock.Lock()
	extDB := db.extDB
	if extDB != nil {
		extDB.incRef()
	}
	db.extDBLock.Unlock()
	if extDB == nil {
		return false
	}
	f(extDB)
	extDB.decRef()
	return true
}

// SetExtDB sets external db to search.
//
// It decrements refCount for the previous extDB.
func (db *indexDB) SetExtDB(extDB *indexDB) {
	// Add deleted metricIDs from extDB to db.
	if extDB != nil {
		dmisExt := extDB.getDeletedMetricIDs()
		db.updateDeletedMetricIDs(dmisExt)
	}

	db.extDBLock.Lock()
	prevExtDB := db.extDB
	db.extDB = extDB
	db.extDBLock.Unlock()

	if prevExtDB != nil {
		prevExtDB.decRef()
	}
}

// MustClose closes db.
func (db *indexDB) MustClose() {
	db.decRef()
}

func (db *indexDB) incRef() {
	atomic.AddUint64(&db.refCount, 1)
}

func (db *indexDB) decRef() {
	n := atomic.AddUint64(&db.refCount, ^uint64(0))
	if int64(n) < 0 {
		logger.Panicf("BUG: negative refCount: %d", n)
	}
	if n > 0 {
		return
	}

	tbPath := db.tb.Path()
	db.tb.MustClose()
	db.SetExtDB(nil)

	// Free space occupied by caches owned by db.
	db.tagCache.Stop()
	db.uselessTagFiltersCache.Stop()
	db.metricIDsPerDateTagFilterCache.Stop()

	db.tagCache = nil
	db.metricIDCache = nil
	db.metricNameCache = nil
	db.tsidCache = nil
	db.uselessTagFiltersCache = nil
	db.metricIDsPerDateTagFilterCache = nil

	if atomic.LoadUint64(&db.mustDrop) == 0 {
		return
	}

	logger.Infof("dropping indexDB %q", tbPath)
	fs.MustRemoveAll(tbPath)
	logger.Infof("indexDB %q has been dropped", tbPath)
}

func (db *indexDB) getFromTagCache(key []byte) ([]TSID, bool) {
	compressedBuf := tagBufPool.Get()
	defer tagBufPool.Put(compressedBuf)
	compressedBuf.B = db.tagCache.GetBig(compressedBuf.B[:0], key)
	if len(compressedBuf.B) == 0 {
		return nil, false
	}
	buf := tagBufPool.Get()
	defer tagBufPool.Put(buf)
	var err error
	buf.B, err = encoding.DecompressZSTD(buf.B[:0], compressedBuf.B)
	if err != nil {
		logger.Panicf("FATAL: cannot decompress tsids from tagCache: %s", err)
	}
	tsids, err := unmarshalTSIDs(nil, buf.B)
	if err != nil {
		logger.Panicf("FATAL: cannot unmarshal tsids from tagCache: %s", err)
	}
	return tsids, true
}

var tagBufPool bytesutil.ByteBufferPool

func (db *indexDB) putToTagCache(tsids []TSID, key []byte) {
	buf := tagBufPool.Get()
	buf.B = marshalTSIDs(buf.B[:0], tsids)
	compressedBuf := tagBufPool.Get()
	compressedBuf.B = encoding.CompressZSTDLevel(compressedBuf.B[:0], buf.B, 1)
	tagBufPool.Put(buf)
	db.tagCache.SetBig(key, compressedBuf.B)
	tagBufPool.Put(compressedBuf)
}

func (db *indexDB) getFromMetricIDCache(dst *TSID, metricID uint64) error {
	// There is no need in checking for deleted metricIDs here, since they
	// must be checked by the caller.
	buf := (*[unsafe.Sizeof(*dst)]byte)(unsafe.Pointer(dst))
	key := (*[unsafe.Sizeof(metricID)]byte)(unsafe.Pointer(&metricID))
	tmp := db.metricIDCache.Get(buf[:0], key[:])
	if len(tmp) == 0 {
		// The TSID for the given metricID wasn't found in the cache.
		return io.EOF
	}
	if &tmp[0] != &buf[0] || len(tmp) != len(buf) {
		return fmt.Errorf("corrupted MetricID->TSID cache: unexpected size for metricID=%d value; got %d bytes; want %d bytes", metricID, len(tmp), len(buf))
	}
	return nil
}

func (db *indexDB) putToMetricIDCache(metricID uint64, tsid *TSID) {
	buf := (*[unsafe.Sizeof(*tsid)]byte)(unsafe.Pointer(tsid))
	key := (*[unsafe.Sizeof(metricID)]byte)(unsafe.Pointer(&metricID))
	db.metricIDCache.Set(key[:], buf[:])
}

func (db *indexDB) getMetricNameFromCache(dst []byte, metricID uint64) []byte {
	// There is no need in checking for deleted metricIDs here, since they
	// must be checked by the caller.
	key := (*[unsafe.Sizeof(metricID)]byte)(unsafe.Pointer(&metricID))
	return db.metricNameCache.Get(dst, key[:])
}

func (db *indexDB) putMetricNameToCache(metricID uint64, metricName []byte) {
	key := (*[unsafe.Sizeof(metricID)]byte)(unsafe.Pointer(&metricID))
	db.metricNameCache.Set(key[:], metricName)
}

func marshalTagFiltersKey(dst []byte, tfss []*TagFilters, tr TimeRange, versioned bool) []byte {
	prefix := ^uint64(0)
	if versioned {
		prefix = atomic.LoadUint64(&tagFiltersKeyGen)
	}
	const cacheGranularityMs = 1000 * 10
	startTime := (uint64(tr.MinTimestamp) / cacheGranularityMs) * cacheGranularityMs
	endTime := (uint64(tr.MaxTimestamp) / cacheGranularityMs) * cacheGranularityMs
	dst = encoding.MarshalUint64(dst, prefix)
	dst = encoding.MarshalUint64(dst, startTime)
	dst = encoding.MarshalUint64(dst, endTime)
	for _, tfs := range tfss {
		dst = append(dst, 0) // separator between tfs groups.
		for i := range tfs.tfs {
			dst = tfs.tfs[i].Marshal(dst)
		}
	}
	return dst
}

func marshalTSIDs(dst []byte, tsids []TSID) []byte {
	dst = encoding.MarshalUint64(dst, uint64(len(tsids)))
	for i := range tsids {
		dst = tsids[i].Marshal(dst)
	}
	return dst
}

func unmarshalTSIDs(dst []TSID, src []byte) ([]TSID, error) {
	if len(src) < 8 {
		return dst, fmt.Errorf("cannot unmarshal the number of tsids from %d bytes; require at least %d bytes", len(src), 8)
	}
	n := encoding.UnmarshalUint64(src)
	src = src[8:]
	dstLen := len(dst)
	if nn := dstLen + int(n) - cap(dst); nn > 0 {
		dst = append(dst[:cap(dst)], make([]TSID, nn)...)
	}
	dst = dst[:dstLen+int(n)]
	for i := 0; i < int(n); i++ {
		tail, err := dst[dstLen+i].Unmarshal(src)
		if err != nil {
			return dst, fmt.Errorf("cannot unmarshal tsid #%d out of %d: %w", i, n, err)
		}
		src = tail
	}
	if len(src) > 0 {
		return dst, fmt.Errorf("non-zero tail left after unmarshaling %d tsids; len(tail)=%d", n, len(src))
	}
	return dst, nil
}

func invalidateTagCache() {
	// This function must be fast, since it is called each
	// time new timeseries is added.
	atomic.AddUint64(&tagFiltersKeyGen, 1)
}

var tagFiltersKeyGen uint64

// getTSIDByNameNoCreate fills the dst with TSID for the given metricName.
//
// It returns io.EOF if the given mn isn't found locally.
func (db *indexDB) getTSIDByNameNoCreate(dst *TSID, metricName []byte) error {
	is := db.getIndexSearch()
	err := is.getTSIDByMetricName(dst, metricName)
	db.putIndexSearch(is)
	if err == nil {
		return nil
	}
	if err != io.EOF {
		return fmt.Errorf("cannot search TSID by MetricName %q: %w", metricName, err)
	}

	// Do not search for the TSID in the external storage,
	// since this function is already called by another indexDB instance.

	// The TSID for the given mn wasn't found.
	return io.EOF
}

type indexSearch struct {
	db *indexDB
	ts mergeset.TableSearch
	kb bytesutil.ByteBuffer
	mp tagToMetricIDsRowParser

	// tsidByNameMisses and tsidByNameSkips is used for a performance
	// hack in GetOrCreateTSIDByName. See the comment there.
	tsidByNameMisses int
	tsidByNameSkips  int
}

// GetOrCreateTSIDByName fills the dst with TSID for the given metricName.
func (is *indexSearch) GetOrCreateTSIDByName(dst *TSID, metricName []byte) error {
	// A hack: skip searching for the TSID after many serial misses.
	// This should improve insertion performance for big batches
	// of new time series.
	if is.tsidByNameMisses < 100 {
		err := is.getTSIDByMetricName(dst, metricName)
		if err == nil {
			is.tsidByNameMisses = 0
			return nil
		}
		if err != io.EOF {
			return fmt.Errorf("cannot search TSID by MetricName %q: %w", metricName, err)
		}
		is.tsidByNameMisses++
	} else {
		is.tsidByNameSkips++
		if is.tsidByNameSkips > 10000 {
			is.tsidByNameSkips = 0
			is.tsidByNameMisses = 0
		}
	}

	// TSID for the given name wasn't found. Create it.
	// It is OK if duplicate TSID for mn is created by concurrent goroutines.
	// Metric results will be merged by mn after TableSearch.
	if err := is.db.createTSIDByName(dst, metricName); err != nil {
		return fmt.Errorf("cannot create TSID by MetricName %q: %w", metricName, err)
	}
	return nil
}

func (db *indexDB) getIndexSearch() *indexSearch {
	v := db.indexSearchPool.Get()
	if v == nil {
		v = &indexSearch{
			db: db,
		}
	}
	is := v.(*indexSearch)
	is.ts.Init(db.tb, shouldCacheBlock)
	return is
}

func (db *indexDB) putIndexSearch(is *indexSearch) {
	is.ts.MustClose()
	is.kb.Reset()
	is.mp.Reset()

	// Do not reset tsidByNameMisses and tsidByNameSkips,
	// since they are used in GetOrCreateTSIDByName across call boundaries.

	db.indexSearchPool.Put(is)
}

func (db *indexDB) createTSIDByName(dst *TSID, metricName []byte) error {
	mn := GetMetricName()
	defer PutMetricName(mn)
	if err := mn.Unmarshal(metricName); err != nil {
		return fmt.Errorf("cannot unmarshal metricName %q: %w", metricName, err)
	}

	if err := db.generateTSID(dst, metricName, mn); err != nil {
		return fmt.Errorf("cannot generate TSID: %w", err)
	}
	db.putMetricNameToCache(dst.MetricID, metricName)
	if err := db.createIndexes(dst, mn); err != nil {
		return fmt.Errorf("cannot create indexes: %w", err)
	}

	// There is no need in invalidating tag cache, since it is invalidated
	// on db.tb flush via invalidateTagCache flushCallback passed to OpenTable.

	atomic.AddUint64(&db.newTimeseriesCreated, 1)
	return nil
}

func (db *indexDB) generateTSID(dst *TSID, metricName []byte, mn *MetricName) error {
	// Search the TSID in the external storage.
	// This is usually the db from the previous period.
	var err error
	if db.doExtDB(func(extDB *indexDB) {
		err = extDB.getTSIDByNameNoCreate(dst, metricName)
	}) {
		if err == nil {
			// The TSID has been found in the external storage.
			return nil
		}
		if err != io.EOF {
			return fmt.Errorf("external search failed: %w", err)
		}
	}

	// The TSID wasn't found in the external storage.
	// Generate it locally.
	dst.MetricGroupID = xxhash.Sum64(mn.MetricGroup)
	if len(mn.Tags) > 0 {
		dst.JobID = uint32(xxhash.Sum64(mn.Tags[0].Value))
	}
	if len(mn.Tags) > 1 {
		dst.InstanceID = uint32(xxhash.Sum64(mn.Tags[1].Value))
	}
	dst.MetricID = generateUniqueMetricID()
	return nil
}

func (db *indexDB) createIndexes(tsid *TSID, mn *MetricName) error {
	// The order of index items is important.
	// It guarantees index consistency.

	items := getIndexItems()

	// Create MetricName -> TSID index.
	items.B = append(items.B, nsPrefixMetricNameToTSID)
	items.B = mn.Marshal(items.B)
	items.B = append(items.B, kvSeparatorChar)
	items.B = tsid.Marshal(items.B)
	items.Next()

	// Create MetricID -> MetricName index.
	items.B = marshalCommonPrefix(items.B, nsPrefixMetricIDToMetricName)
	items.B = encoding.MarshalUint64(items.B, tsid.MetricID)
	items.B = mn.Marshal(items.B)
	items.Next()

	// Create MetricID -> TSID index.
	items.B = marshalCommonPrefix(items.B, nsPrefixMetricIDToTSID)
	items.B = encoding.MarshalUint64(items.B, tsid.MetricID)
	items.B = tsid.Marshal(items.B)
	items.Next()

	commonPrefix := kbPool.Get()
	commonPrefix.B = marshalCommonPrefix(commonPrefix.B[:0], nsPrefixTagToMetricIDs)

	// Create MetricGroup -> MetricID index.
	items.B = append(items.B, commonPrefix.B...)
	items.B = marshalTagValue(items.B, nil)
	items.B = marshalTagValue(items.B, mn.MetricGroup)
	items.B = encoding.MarshalUint64(items.B, tsid.MetricID)
	items.Next()
	addReverseMetricGroupIfNeeded(items, commonPrefix.B, mn, tsid.MetricID)

	// For each tag create tag -> MetricID index.
	for i := range mn.Tags {
		tag := &mn.Tags[i]
		items.B = append(items.B, commonPrefix.B...)
		items.B = tag.Marshal(items.B)
		items.B = encoding.MarshalUint64(items.B, tsid.MetricID)
		items.Next()
	}

	kbPool.Put(commonPrefix)
	err := db.tb.AddItems(items.Items)
	putIndexItems(items)
	return err
}

type indexItems struct {
	B     []byte
	Items [][]byte

	start int
}

func (ii *indexItems) reset() {
	ii.B = ii.B[:0]
	ii.Items = ii.Items[:0]
	ii.start = 0
}

func (ii *indexItems) Next() {
	ii.Items = append(ii.Items, ii.B[ii.start:])
	ii.start = len(ii.B)
}

func getIndexItems() *indexItems {
	v := indexItemsPool.Get()
	if v == nil {
		return &indexItems{}
	}
	return v.(*indexItems)
}

func putIndexItems(ii *indexItems) {
	ii.reset()
	indexItemsPool.Put(ii)
}

var indexItemsPool sync.Pool

// SearchTagKeys returns all the tag keys.
func (db *indexDB) SearchTagKeys(maxTagKeys int) ([]string, error) {
	// TODO: cache results?

	tks := make(map[string]struct{})

	is := db.getIndexSearch()
	err := is.searchTagKeys(tks, maxTagKeys)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}

	ok := db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch()
		err = is.searchTagKeys(tks, maxTagKeys)
		extDB.putIndexSearch(is)
	})
	if ok && err != nil {
		return nil, err
	}

	keys := make([]string, 0, len(tks))
	for key := range tks {
		// Do not skip empty keys, since they are converted to __name__
		keys = append(keys, key)
	}

	// Do not sort keys, since they must be sorted by vmselect.
	return keys, nil
}

func (is *indexSearch) searchTagKeys(tks map[string]struct{}, maxTagKeys int) error {
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	mp.Reset()
	dmis := is.db.getDeletedMetricIDs()
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	prefix := kb.B
	ts.Seek(prefix)
	for len(tks) < maxTagKeys && ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		if err := mp.Init(item, nsPrefixTagToMetricIDs); err != nil {
			return err
		}
		if mp.IsDeletedTag(dmis) {
			continue
		}

		// Store tag key.
		tks[string(mp.Tag.Key)] = struct{}{}

		// Search for the next tag key.
		// The last char in kb.B must be tagSeparatorChar.
		// Just increment it in order to jump to the next tag key.
		kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
		kb.B = marshalTagValue(kb.B, mp.Tag.Key)
		kb.B[len(kb.B)-1]++
		ts.Seek(kb.B)
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error during search for prefix %q: %w", prefix, err)
	}
	return nil
}

// SearchTagValues returns all the tag values for the given tagKey
func (db *indexDB) SearchTagValues(tagKey []byte, maxTagValues int) ([]string, error) {
	// TODO: cache results?

	tvs := make(map[string]struct{})
	is := db.getIndexSearch()
	err := is.searchTagValues(tvs, tagKey, maxTagValues)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}
	ok := db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch()
		err = is.searchTagValues(tvs, tagKey, maxTagValues)
		extDB.putIndexSearch(is)
	})
	if ok && err != nil {
		return nil, err
	}

	tagValues := make([]string, 0, len(tvs))
	for tv := range tvs {
		if len(tv) == 0 {
			// Skip empty values, since they have no any meaning.
			// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/600
			continue
		}
		tagValues = append(tagValues, tv)
	}

	// Do not sort tagValues, since they must be sorted by vmselect.
	return tagValues, nil
}

func (is *indexSearch) searchTagValues(tvs map[string]struct{}, tagKey []byte, maxTagValues int) error {
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	mp.Reset()
	dmis := is.db.getDeletedMetricIDs()
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	kb.B = marshalTagValue(kb.B, tagKey)
	prefix := kb.B
	ts.Seek(prefix)
	for len(tvs) < maxTagValues && ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		if err := mp.Init(item, nsPrefixTagToMetricIDs); err != nil {
			return err
		}
		if mp.IsDeletedTag(dmis) {
			continue
		}

		// Store tag value
		tvs[string(mp.Tag.Value)] = struct{}{}

		if mp.MetricIDsLen() < maxMetricIDsPerRow/2 {
			// There is no need in searching for the next tag value,
			// since it is likely it is located in the next row,
			// because the current row contains incomplete metricIDs set.
			continue
		}
		// Search for the next tag value.
		// The last char in kb.B must be tagSeparatorChar.
		// Just increment it in order to jump to the next tag value.
		kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
		kb.B = marshalTagValue(kb.B, mp.Tag.Key)
		kb.B = marshalTagValue(kb.B, mp.Tag.Value)
		kb.B[len(kb.B)-1]++
		ts.Seek(kb.B)
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for tag name prefix %q: %w", prefix, err)
	}
	return nil
}

// GetSeriesCount returns the approximate number of unique timeseries in the db.
//
// It includes the deleted series too and may count the same series
// up to two times - in db and extDB.
func (db *indexDB) GetSeriesCount() (uint64, error) {
	is := db.getIndexSearch()
	n, err := is.getSeriesCount()
	db.putIndexSearch(is)
	if err != nil {
		return 0, err
	}

	var nExt uint64
	ok := db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch()
		nExt, err = is.getSeriesCount()
		extDB.putIndexSearch(is)
	})
	if ok && err != nil {
		return 0, fmt.Errorf("error when searching in extDB: %w", err)
	}
	return n + nExt, nil
}

func (is *indexSearch) getSeriesCount() (uint64, error) {
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	var metricIDsLen uint64
	// Extract the number of series from ((__name__=value): metricIDs) rows
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	kb.B = marshalTagValue(kb.B, nil)
	ts.Seek(kb.B)
	for ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, kb.B) {
			break
		}
		tail := item[len(kb.B):]
		n := bytes.IndexByte(tail, tagSeparatorChar)
		if n < 0 {
			return 0, fmt.Errorf("invalid tag->metricIDs line %q: cannot find tagSeparatorChar %d", item, tagSeparatorChar)
		}
		tail = tail[n+1:]
		if err := mp.InitOnlyTail(item, tail); err != nil {
			return 0, err
		}
		// Take into account deleted timeseries too.
		// It is OK if series can be counted multiple times in rare cases -
		// the returned number is an estimation.
		metricIDsLen += uint64(mp.MetricIDsLen())
	}
	if err := ts.Error(); err != nil {
		return 0, fmt.Errorf("error when counting unique timeseries: %w", err)
	}
	return metricIDsLen, nil
}

// GetTSDBStatusForDate returns topN entries for tsdb status for the given date.
func (db *indexDB) GetTSDBStatusForDate(date uint64, topN int) (*TSDBStatus, error) {
	is := db.getIndexSearch()
	status, err := is.getTSDBStatusForDate(date, topN)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}
	if status.hasEntries() {
		// The entries were found in the db. There is no need in searching them in extDB.
		return status, nil
	}

	// The entries weren't found in the db. Try searching them in extDB.
	ok := db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch()
		status, err = is.getTSDBStatusForDate(date, topN)
		extDB.putIndexSearch(is)
	})
	if ok && err != nil {
		return nil, fmt.Errorf("error when obtaining TSDB status from extDB: %w", err)
	}
	return status, nil
}

func (is *indexSearch) getTSDBStatusForDate(date uint64, topN int) (*TSDBStatus, error) {
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	thLabelValueCountByLabelName := newTopHeap(topN)
	thSeriesCountByLabelValuePair := newTopHeap(topN)
	thSeriesCountByMetricName := newTopHeap(topN)
	var tmp, labelName, labelNameValue []byte
	var labelValueCountByLabelName, seriesCountByLabelValuePair uint64
	nameEqualBytes := []byte("__name__=")

	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
	kb.B = encoding.MarshalUint64(kb.B, date)
	prefix := kb.B
	ts.Seek(prefix)
	for ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		tail := item[len(prefix):]
		var err error
		tail, tmp, err = unmarshalTagValue(tmp[:0], tail)
		if err != nil {
			return nil, fmt.Errorf("cannot unmarshal tag key from line %q: %w", item, err)
		}
		if len(tmp) == 0 {
			tmp = append(tmp, "__name__"...)
		}
		if !bytes.Equal(tmp, labelName) {
			thLabelValueCountByLabelName.pushIfNonEmpty(labelName, labelValueCountByLabelName)
			labelValueCountByLabelName = 0
			labelName = append(labelName[:0], tmp...)
		}
		tmp = append(tmp, '=')
		tail, tmp, err = unmarshalTagValue(tmp, tail)
		if err != nil {
			return nil, fmt.Errorf("cannot unmarshal tag value from line %q: %w", item, err)
		}
		if !bytes.Equal(tmp, labelNameValue) {
			thSeriesCountByLabelValuePair.pushIfNonEmpty(labelNameValue, seriesCountByLabelValuePair)
			if bytes.HasPrefix(labelNameValue, nameEqualBytes) {
				thSeriesCountByMetricName.pushIfNonEmpty(labelNameValue[len(nameEqualBytes):], seriesCountByLabelValuePair)
			}
			seriesCountByLabelValuePair = 0
			labelValueCountByLabelName++
			labelNameValue = append(labelNameValue[:0], tmp...)
		}
		if err := mp.InitOnlyTail(item, tail); err != nil {
			return nil, err
		}
		// Take into account deleted timeseries too.
		// It is OK if series can be counted multiple times in rare cases -
		// the returned number is an estimation.
		seriesCountByLabelValuePair += uint64(mp.MetricIDsLen())
	}
	if err := ts.Error(); err != nil {
		return nil, fmt.Errorf("error when counting time series by metric names: %w", err)
	}
	thLabelValueCountByLabelName.pushIfNonEmpty(labelName, labelValueCountByLabelName)
	thSeriesCountByLabelValuePair.pushIfNonEmpty(labelNameValue, seriesCountByLabelValuePair)
	if bytes.HasPrefix(labelNameValue, nameEqualBytes) {
		thSeriesCountByMetricName.pushIfNonEmpty(labelNameValue[len(nameEqualBytes):], seriesCountByLabelValuePair)
	}
	status := &TSDBStatus{
		SeriesCountByMetricName:     thSeriesCountByMetricName.getSortedResult(),
		LabelValueCountByLabelName:  thLabelValueCountByLabelName.getSortedResult(),
		SeriesCountByLabelValuePair: thSeriesCountByLabelValuePair.getSortedResult(),
	}
	return status, nil
}

// TSDBStatus contains TSDB status data for /api/v1/status/tsdb.
//
// See https://prometheus.io/docs/prometheus/latest/querying/api/#tsdb-stats
type TSDBStatus struct {
	SeriesCountByMetricName     []TopHeapEntry
	LabelValueCountByLabelName  []TopHeapEntry
	SeriesCountByLabelValuePair []TopHeapEntry
}

func (status *TSDBStatus) hasEntries() bool {
	return len(status.SeriesCountByLabelValuePair) > 0
}

// topHeap maintains a heap of topHeapEntries with the maximum TopHeapEntry.n values.
type topHeap struct {
	topN int
	a    []TopHeapEntry
}

// newTopHeap returns topHeap for topN items.
func newTopHeap(topN int) *topHeap {
	return &topHeap{
		topN: topN,
	}
}

// TopHeapEntry represents an entry from `top heap` used in stats.
type TopHeapEntry struct {
	Name  string
	Count uint64
}

func (th *topHeap) pushIfNonEmpty(name []byte, count uint64) {
	if count == 0 {
		return
	}
	if len(th.a) < th.topN {
		th.a = append(th.a, TopHeapEntry{
			Name:  string(name),
			Count: count,
		})
		heap.Fix(th, len(th.a)-1)
		return
	}
	if count <= th.a[0].Count {
		return
	}
	th.a[0] = TopHeapEntry{
		Name:  string(name),
		Count: count,
	}
	heap.Fix(th, 0)
}

func (th *topHeap) getSortedResult() []TopHeapEntry {
	result := append([]TopHeapEntry{}, th.a...)
	sort.Slice(result, func(i, j int) bool {
		a, b := result[i], result[j]
		if a.Count != b.Count {
			return a.Count > b.Count
		}
		return a.Name < b.Name
	})
	return result
}

// heap.Interface implementation for topHeap.

func (th *topHeap) Len() int {
	return len(th.a)
}

func (th *topHeap) Less(i, j int) bool {
	a := th.a
	return a[i].Count < a[j].Count
}

func (th *topHeap) Swap(i, j int) {
	a := th.a
	a[j], a[i] = a[i], a[j]
}

func (th *topHeap) Push(x interface{}) {
	panic(fmt.Errorf("BUG: Push shouldn't be called"))
}

func (th *topHeap) Pop() interface{} {
	panic(fmt.Errorf("BUG: Pop shouldn't be called"))
}

// searchMetricName appends metric name for the given metricID to dst
// and returns the result.
func (db *indexDB) searchMetricName(dst []byte, metricID uint64) ([]byte, error) {
	is := db.getIndexSearch()
	dst, err := is.searchMetricName(dst, metricID)
	db.putIndexSearch(is)

	if err != io.EOF {
		return dst, err
	}

	// Try searching in the external indexDB.
	if db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch()
		dst, err = is.searchMetricName(dst, metricID)
		extDB.putIndexSearch(is)
	}) {
		return dst, err
	}

	// Cannot find MetricName for the given metricID. This may be the case
	// when indexDB contains incomplete set of metricID -> metricName entries
	// after a snapshot or due to unflushed entries.
	atomic.AddUint64(&db.missingMetricNamesForMetricID, 1)

	// Mark the metricID as deleted, so it will be created again when new data point
	// for the given time series will arrive.
	if err := db.deleteMetricIDs([]uint64{metricID}); err != nil {
		return dst, fmt.Errorf("cannot delete metricID for missing metricID->metricName entry; metricID=%d; error: %w", metricID, err)
	}
	return dst, io.EOF
}

// DeleteTSIDs marks as deleted all the TSIDs matching the given tfss.
//
// The caller must reset all the caches which may contain the deleted TSIDs.
//
// Returns the number of metrics deleted.
func (db *indexDB) DeleteTSIDs(tfss []*TagFilters) (int, error) {
	if len(tfss) == 0 {
		return 0, nil
	}

	// Obtain metricIDs to delete.
	tr := TimeRange{
		MinTimestamp: 0,
		MaxTimestamp: (1 << 63) - 1,
	}
	is := db.getIndexSearch()
	metricIDs, err := is.searchMetricIDs(tfss, tr, 2e9)
	db.putIndexSearch(is)
	if err != nil {
		return 0, err
	}
	if err := db.deleteMetricIDs(metricIDs); err != nil {
		return 0, err
	}

	// Delete TSIDs in the extDB.
	deletedCount := len(metricIDs)
	if db.doExtDB(func(extDB *indexDB) {
		var n int
		n, err = extDB.DeleteTSIDs(tfss)
		deletedCount += n
	}) {
		if err != nil {
			return deletedCount, fmt.Errorf("cannot delete tsids in extDB: %w", err)
		}
	}
	return deletedCount, nil
}

func (db *indexDB) deleteMetricIDs(metricIDs []uint64) error {
	if len(metricIDs) == 0 {
		// Nothing to delete
		return nil
	}

	// Mark the found metricIDs as deleted.
	items := getIndexItems()
	for _, metricID := range metricIDs {
		items.B = append(items.B, nsPrefixDeletedMetricID)
		items.B = encoding.MarshalUint64(items.B, metricID)
		items.Next()
	}
	err := db.tb.AddItems(items.Items)
	putIndexItems(items)
	if err != nil {
		return err
	}

	// atomically add deleted metricIDs to an inmemory map.
	dmis := &uint64set.Set{}
	dmis.AddMulti(metricIDs)
	db.updateDeletedMetricIDs(dmis)

	// Reset TagFilters -> TSIDS cache, since it may contain deleted TSIDs.
	invalidateTagCache()

	// Reset MetricName -> TSID cache, since it may contain deleted TSIDs.
	db.tsidCache.Reset()

	// Do not reset uselessTagFiltersCache, since the found metricIDs
	// on cache miss are filtered out later with deletedMetricIDs.
	return nil
}

func (db *indexDB) getDeletedMetricIDs() *uint64set.Set {
	return db.deletedMetricIDs.Load().(*uint64set.Set)
}

func (db *indexDB) setDeletedMetricIDs(dmis *uint64set.Set) {
	db.deletedMetricIDs.Store(dmis)
}

func (db *indexDB) updateDeletedMetricIDs(metricIDs *uint64set.Set) {
	db.deletedMetricIDsUpdateLock.Lock()
	dmisOld := db.getDeletedMetricIDs()
	dmisNew := dmisOld.Clone()
	dmisNew.Union(metricIDs)
	db.setDeletedMetricIDs(dmisNew)
	db.deletedMetricIDsUpdateLock.Unlock()
}

func (is *indexSearch) getStartDateForPerDayInvertedIndex() (uint64, error) {
	minDate := fasttime.UnixDate()
	kb := &is.kb
	ts := &is.ts
	kb.B = append(kb.B[:0], nsPrefixDateTagToMetricIDs)
	prefix := kb.B
	ts.Seek(kb.B)
	if ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			// The databse doesn't contain per-day inverted index yet.
			return minDate, nil
		}
		suffix := item[len(prefix):]

		// Suffix must contain encoded 64-bit date.
		if len(suffix) < 8 {
			return 0, fmt.Errorf("unexpected (date, tag)->metricIDs row len; must be at least 8 bytes; got %d bytes", len(suffix))
		}
		minDate = encoding.UnmarshalUint64(suffix)
		return minDate, nil
	}
	if err := ts.Error(); err != nil {
		return 0, err
	}
	// There are no (date,tag)->metricIDs entries in the database yet.
	return minDate, nil
}

func (is *indexSearch) loadDeletedMetricIDs() (*uint64set.Set, error) {
	dmis := &uint64set.Set{}
	ts := &is.ts
	kb := &is.kb
	kb.B = append(kb.B[:0], nsPrefixDeletedMetricID)
	ts.Seek(kb.B)
	for ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, kb.B) {
			break
		}
		item = item[len(kb.B):]
		if len(item) != 8 {
			return nil, fmt.Errorf("unexpected item len; got %d bytes; want %d bytes", len(item), 8)
		}
		metricID := encoding.UnmarshalUint64(item)
		dmis.Add(metricID)
	}
	if err := ts.Error(); err != nil {
		return nil, err
	}
	return dmis, nil
}

// searchTSIDs returns sorted tsids matching the given tfss over the given tr.
func (db *indexDB) searchTSIDs(tfss []*TagFilters, tr TimeRange, maxMetrics int) ([]TSID, error) {
	if len(tfss) == 0 {
		return nil, nil
	}

	tfKeyBuf := tagFiltersKeyBufPool.Get()
	defer tagFiltersKeyBufPool.Put(tfKeyBuf)

	tfKeyBuf.B = marshalTagFiltersKey(tfKeyBuf.B[:0], tfss, tr, true)
	tsids, ok := db.getFromTagCache(tfKeyBuf.B)
	if ok {
		// Fast path - tsids found in the cache.
		return tsids, nil
	}

	// Slow path - search for tsids in the db and extDB.
	is := db.getIndexSearch()
	localTSIDs, err := is.searchTSIDs(tfss, tr, maxMetrics)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}

	var extTSIDs []TSID
	if db.doExtDB(func(extDB *indexDB) {
		tfKeyExtBuf := tagFiltersKeyBufPool.Get()
		defer tagFiltersKeyBufPool.Put(tfKeyExtBuf)

		// Data in extDB cannot be changed, so use unversioned keys for tag cache.
		tfKeyExtBuf.B = marshalTagFiltersKey(tfKeyExtBuf.B[:0], tfss, tr, false)
		tsids, ok := extDB.getFromTagCache(tfKeyExtBuf.B)
		if ok {
			extTSIDs = tsids
			return
		}
		is := extDB.getIndexSearch()
		extTSIDs, err = is.searchTSIDs(tfss, tr, maxMetrics)
		extDB.putIndexSearch(is)

		sort.Slice(extTSIDs, func(i, j int) bool { return extTSIDs[i].Less(&extTSIDs[j]) })
		extDB.putToTagCache(extTSIDs, tfKeyExtBuf.B)
	}) {
		if err != nil {
			return nil, err
		}
	}

	// Merge localTSIDs with extTSIDs.
	tsids = mergeTSIDs(localTSIDs, extTSIDs)

	// Sort the found tsids, since they must be passed to TSID search
	// in the sorted order.
	sort.Slice(tsids, func(i, j int) bool { return tsids[i].Less(&tsids[j]) })

	// Store TSIDs in the cache.
	db.putToTagCache(tsids, tfKeyBuf.B)

	return tsids, err
}

var tagFiltersKeyBufPool bytesutil.ByteBufferPool

func (is *indexSearch) getTSIDByMetricName(dst *TSID, metricName []byte) error {
	dmis := is.db.getDeletedMetricIDs()
	ts := &is.ts
	kb := &is.kb
	kb.B = append(kb.B[:0], nsPrefixMetricNameToTSID)
	kb.B = append(kb.B, metricName...)
	kb.B = append(kb.B, kvSeparatorChar)
	ts.Seek(kb.B)
	for ts.NextItem() {
		if !bytes.HasPrefix(ts.Item, kb.B) {
			// Nothing found.
			return io.EOF
		}
		v := ts.Item[len(kb.B):]
		tail, err := dst.Unmarshal(v)
		if err != nil {
			return fmt.Errorf("cannot unmarshal TSID: %w", err)
		}
		if len(tail) > 0 {
			return fmt.Errorf("unexpected non-empty tail left after unmarshaling TSID: %X", tail)
		}
		if dmis.Len() > 0 {
			// Verify whether the dst is marked as deleted.
			if dmis.Has(dst.MetricID) {
				// The dst is deleted. Continue searching.
				continue
			}
		}
		// Found valid dst.
		return nil
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching TSID by metricName; searchPrefix %q: %w", kb.B, err)
	}
	// Nothing found
	return io.EOF
}

func (is *indexSearch) searchMetricName(dst []byte, metricID uint64) ([]byte, error) {
	metricName := is.db.getMetricNameFromCache(dst, metricID)
	if len(metricName) > len(dst) {
		return metricName, nil
	}

	ts := &is.ts
	kb := &is.kb
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixMetricIDToMetricName)
	kb.B = encoding.MarshalUint64(kb.B, metricID)
	if err := ts.FirstItemWithPrefix(kb.B); err != nil {
		if err == io.EOF {
			return dst, err
		}
		return dst, fmt.Errorf("error when searching metricName by metricID; searchPrefix %q: %w", kb.B, err)
	}
	v := ts.Item[len(kb.B):]
	dst = append(dst, v...)

	// There is no need in verifying whether the given metricID is deleted,
	// since the filtering must be performed before calling this func.
	is.db.putMetricNameToCache(metricID, dst)
	return dst, nil
}

func mergeTSIDs(a, b []TSID) []TSID {
	if len(b) > len(a) {
		a, b = b, a
	}
	if len(b) == 0 {
		return a
	}
	m := make(map[uint64]TSID, len(a))
	for i := range a {
		tsid := &a[i]
		m[tsid.MetricID] = *tsid
	}
	for i := range b {
		tsid := &b[i]
		m[tsid.MetricID] = *tsid
	}

	tsids := make([]TSID, 0, len(m))
	for _, tsid := range m {
		tsids = append(tsids, tsid)
	}
	return tsids
}

func (is *indexSearch) containsTimeRange(tr TimeRange) (bool, error) {
	ts := &is.ts
	kb := &is.kb

	// Verify whether the maximum date in `ts` covers tr.MinTimestamp.
	minDate := uint64(tr.MinTimestamp) / msecPerDay
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixDateToMetricID)
	prefix := kb.B
	kb.B = encoding.MarshalUint64(kb.B, minDate)
	ts.Seek(kb.B)
	if !ts.NextItem() {
		if err := ts.Error(); err != nil {
			return false, fmt.Errorf("error when searching for minDate=%d, prefix %q: %w", minDate, kb.B, err)
		}
		return false, nil
	}
	if !bytes.HasPrefix(ts.Item, prefix) {
		// minDate exceeds max date from ts.
		return false, nil
	}
	return true, nil
}

func (is *indexSearch) searchTSIDs(tfss []*TagFilters, tr TimeRange, maxMetrics int) ([]TSID, error) {
	ok, err := is.containsTimeRange(tr)
	if err != nil {
		return nil, err
	}
	if !ok {
		// Fast path - the index doesn't contain data for the given tr.
		return nil, nil
	}
	metricIDs, err := is.searchMetricIDs(tfss, tr, maxMetrics)
	if err != nil {
		return nil, err
	}
	if len(metricIDs) == 0 {
		// Nothing found.
		return nil, nil
	}

	// Obtain TSID values for the given metricIDs.
	tsids := make([]TSID, len(metricIDs))
	i := 0
	for _, metricID := range metricIDs {
		// Try obtaining TSIDs from MetricID->TSID cache. This is much faster
		// than scanning the mergeset if it contains a lot of metricIDs.
		tsid := &tsids[i]
		err := is.db.getFromMetricIDCache(tsid, metricID)
		if err == nil {
			// Fast path - the tsid for metricID is found in cache.
			i++
			continue
		}
		if err != io.EOF {
			return nil, err
		}
		if err := is.getTSIDByMetricID(tsid, metricID); err != nil {
			if err == io.EOF {
				// Cannot find TSID for the given metricID.
				// This may be the case on incomplete indexDB
				// due to snapshot or due to unflushed entries.
				// Just increment errors counter and skip it.
				atomic.AddUint64(&is.db.missingTSIDsForMetricID, 1)
				continue
			}
			return nil, fmt.Errorf("cannot find tsid %d out of %d for metricID %d: %w", i, len(metricIDs), metricID, err)
		}
		is.db.putToMetricIDCache(metricID, tsid)
		i++
	}
	tsids = tsids[:i]

	// Do not sort the found tsids, since they will be sorted later.
	return tsids, nil
}

func (is *indexSearch) getTSIDByMetricID(dst *TSID, metricID uint64) error {
	// There is no need in checking for deleted metricIDs here, since they
	// must be checked by the caller.
	ts := &is.ts
	kb := &is.kb
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixMetricIDToTSID)
	kb.B = encoding.MarshalUint64(kb.B, metricID)
	if err := ts.FirstItemWithPrefix(kb.B); err != nil {
		if err == io.EOF {
			return err
		}
		return fmt.Errorf("error when searching TSID by metricID; searchPrefix %q: %w", kb.B, err)
	}
	v := ts.Item[len(kb.B):]
	tail, err := dst.Unmarshal(v)
	if err != nil {
		return fmt.Errorf("cannot unmarshal TSID=%X: %w", v, err)
	}
	if len(tail) > 0 {
		return fmt.Errorf("unexpected non-zero tail left after unmarshaling TSID: %X", tail)
	}
	return nil
}

// updateMetricIDsByMetricNameMatch matches metricName values for the given srcMetricIDs against tfs
// and adds matching metrics to metricIDs.
func (is *indexSearch) updateMetricIDsByMetricNameMatch(metricIDs, srcMetricIDs *uint64set.Set, tfs []*tagFilter) error {
	// sort srcMetricIDs in order to speed up Seek below.
	sortedMetricIDs := srcMetricIDs.AppendTo(nil)

	metricName := kbPool.Get()
	defer kbPool.Put(metricName)
	mn := GetMetricName()
	defer PutMetricName(mn)
	for _, metricID := range sortedMetricIDs {
		var err error
		metricName.B, err = is.searchMetricName(metricName.B[:0], metricID)
		if err != nil {
			if err == io.EOF {
				// It is likely the metricID->metricName entry didn't propagate to inverted index yet.
				// Skip this metricID for now.
				continue
			}
			return fmt.Errorf("cannot find metricName by metricID %d: %w", metricID, err)
		}
		if err := mn.Unmarshal(metricName.B); err != nil {
			return fmt.Errorf("cannot unmarshal metricName %q: %w", metricName.B, err)
		}

		// Match the mn against tfs.
		ok, err := matchTagFilters(mn, tfs, &is.kb)
		if err != nil {
			return fmt.Errorf("cannot match MetricName %s against tagFilters: %w", mn, err)
		}
		if !ok {
			continue
		}
		metricIDs.Add(metricID)
	}
	return nil
}

func (is *indexSearch) getTagFilterWithMinMetricIDsCountOptimized(tfs *TagFilters, tr TimeRange, maxMetrics int) (*tagFilter, *uint64set.Set, error) {
	// Try fast path with the minimized number of maxMetrics.
	maxMetricsAdjusted := is.adjustMaxMetricsAdaptive(tr, maxMetrics)
	minTf, minMetricIDs, err := is.getTagFilterWithMinMetricIDsCountAdaptive(tfs, maxMetricsAdjusted)
	if err == nil {
		return minTf, minMetricIDs, nil
	}
	if err != errTooManyMetrics {
		return nil, nil, err
	}

	// All the tag filters match too many metrics.

	// Slow path: try filtering the matching metrics by time range.
	// This should work well for cases when old metrics are constantly substituted
	// by big number of new metrics. For example, prometheus-operator creates many new
	// metrics for each new deployment.
	//
	// Allow fetching up to 20*maxMetrics metrics for the given time range
	// in the hope these metricIDs will be filtered out by other filters later.
	maxTimeRangeMetrics := 20 * maxMetrics
	metricIDsForTimeRange, err := is.getMetricIDsForTimeRange(tr, maxTimeRangeMetrics+1)
	if err == errMissingMetricIDsForDate {
		// Slow path: try to find the tag filter without maxMetrics adjustement.
		minTf, minMetricIDs, err = is.getTagFilterWithMinMetricIDsCountAdaptive(tfs, maxMetrics)
		if err == nil {
			return minTf, minMetricIDs, nil
		}
		if err != errTooManyMetrics {
			return nil, nil, err
		}
		return nil, nil, fmt.Errorf("cannot find tag filter matching less than %d time series; "+
			"either increase -search.maxUniqueTimeseries or use more specific tag filters", maxMetrics)
	}
	if err != nil {
		return nil, nil, err
	}
	if metricIDsForTimeRange.Len() <= maxTimeRangeMetrics {
		return nil, metricIDsForTimeRange, nil
	}

	// Slow path: try to select the tag filter without maxMetrics adjustement.
	minTf, minMetricIDs, err = is.getTagFilterWithMinMetricIDsCountAdaptive(tfs, maxMetrics)
	if err == nil {
		return minTf, minMetricIDs, nil
	}
	if err != errTooManyMetrics {
		return nil, nil, err
	}
	return nil, nil, fmt.Errorf("more than %d time series found on the time range %s; either increase -search.maxUniqueTimeseries or shrink the time range",
		maxMetrics, tr.String())
}

const maxDaysForDateMetricIDs = 40

func (is *indexSearch) adjustMaxMetricsAdaptive(tr TimeRange, maxMetrics int) int {
	minDate := uint64(tr.MinTimestamp) / msecPerDay
	maxDate := uint64(tr.MaxTimestamp) / msecPerDay
	if maxDate-minDate > maxDaysForDateMetricIDs {
		// Cannot reduce maxMetrics for the given time range,
		// since it is expensive extracting metricIDs for the given tr.
		return maxMetrics
	}
	hmPrev := is.db.prevHourMetricIDs.Load().(*hourMetricIDs)
	if !hmPrev.isFull {
		return maxMetrics
	}
	hourMetrics := hmPrev.m.Len()
	if maxMetrics > hourMetrics {
		// It is cheaper to filter on the hour or day metrics if the minimum
		// number of matching metrics across tfs exceeds hourMetrics.
		return hourMetrics
	}
	return maxMetrics
}

func (is *indexSearch) getTagFilterWithMinMetricIDsCountAdaptive(tfs *TagFilters, maxMetrics int) (*tagFilter, *uint64set.Set, error) {
	kb := &is.kb
	kb.B = append(kb.B[:0], uselessMultiTagFiltersKeyPrefix)
	kb.B = encoding.MarshalUint64(kb.B, uint64(maxMetrics))
	kb.B = tfs.marshal(kb.B)
	if len(is.db.uselessTagFiltersCache.Get(nil, kb.B)) > 0 {
		// Skip useless work below, since the tfs doesn't contain tag filters matching less than maxMetrics metrics.
		return nil, nil, errTooManyMetrics
	}

	// Iteratively increase maxAllowedMetrics up to maxMetrics in order to limit
	// the time required for founding the tag filter with minimum matching metrics.
	maxAllowedMetrics := 16
	if maxAllowedMetrics > maxMetrics {
		maxAllowedMetrics = maxMetrics
	}
	for {
		minTf, minMetricIDs, err := is.getTagFilterWithMinMetricIDsCount(tfs, maxAllowedMetrics)
		if err != errTooManyMetrics {
			if err != nil {
				return nil, nil, err
			}
			if minMetricIDs.Len() < maxAllowedMetrics {
				// Found the tag filter with the minimum number of metrics.
				return minTf, minMetricIDs, nil
			}
		}

		// Too many metrics matched.
		if maxAllowedMetrics >= maxMetrics {
			// The tag filter with minimum matching metrics matches at least maxMetrics metrics.
			kb.B = append(kb.B[:0], uselessMultiTagFiltersKeyPrefix)
			kb.B = encoding.MarshalUint64(kb.B, uint64(maxMetrics))
			kb.B = tfs.marshal(kb.B)
			is.db.uselessTagFiltersCache.Set(kb.B, uselessTagFilterCacheValue)
			return nil, nil, errTooManyMetrics
		}

		// Increase maxAllowedMetrics and try again.
		maxAllowedMetrics *= 4
		if maxAllowedMetrics > maxMetrics {
			maxAllowedMetrics = maxMetrics
		}
	}
}

var errTooManyMetrics = errors.New("all the tag filters match too many metrics")

func (is *indexSearch) getTagFilterWithMinMetricIDsCount(tfs *TagFilters, maxMetrics int) (*tagFilter, *uint64set.Set, error) {
	var minMetricIDs *uint64set.Set
	var minTf *tagFilter
	kb := &is.kb
	uselessTagFilters := 0
	for i := range tfs.tfs {
		tf := &tfs.tfs[i]
		if tf.isNegative {
			// Skip negative filters.
			continue
		}

		kb.B = append(kb.B[:0], uselessSingleTagFilterKeyPrefix)
		kb.B = encoding.MarshalUint64(kb.B, uint64(maxMetrics))
		kb.B = tf.Marshal(kb.B)
		if len(is.db.uselessTagFiltersCache.Get(nil, kb.B)) > 0 {
			// Skip useless work below, since the tf matches at least maxMetrics metrics.
			uselessTagFilters++
			continue
		}

		metricIDs, err := is.getMetricIDsForTagFilter(tf, maxMetrics)
		if err != nil {
			if err == errFallbackToMetricNameMatch {
				// Skip tag filters requiring to scan for too many metrics.
				kb.B = append(kb.B[:0], uselessSingleTagFilterKeyPrefix)
				kb.B = encoding.MarshalUint64(kb.B, uint64(maxMetrics))
				kb.B = tf.Marshal(kb.B)
				is.db.uselessTagFiltersCache.Set(kb.B, uselessTagFilterCacheValue)
				uselessTagFilters++
				continue
			}
			return nil, nil, fmt.Errorf("cannot find MetricIDs for tagFilter %s: %w", tf, err)
		}
		if metricIDs.Len() >= maxMetrics {
			// The tf matches at least maxMetrics. Skip it
			kb.B = append(kb.B[:0], uselessSingleTagFilterKeyPrefix)
			kb.B = encoding.MarshalUint64(kb.B, uint64(maxMetrics))
			kb.B = tf.Marshal(kb.B)
			is.db.uselessTagFiltersCache.Set(kb.B, uselessTagFilterCacheValue)
			uselessTagFilters++
			continue
		}

		minMetricIDs = metricIDs
		minTf = tf
		maxMetrics = minMetricIDs.Len()
		if maxMetrics <= 1 {
			// There is no need in inspecting other filters, since minTf
			// already matches 0 or 1 metric.
			break
		}
	}
	if minTf != nil {
		return minTf, minMetricIDs, nil
	}
	if uselessTagFilters == len(tfs.tfs) {
		// All the tag filters return at least maxMetrics entries.
		return nil, nil, errTooManyMetrics
	}

	// There is no positive filter with small number of matching metrics.
	// Create it, so it matches all the MetricIDs.
	kb.B = append(kb.B[:0], uselessNegativeTagFilterKeyPrefix)
	kb.B = encoding.MarshalUint64(kb.B, uint64(maxMetrics))
	kb.B = tfs.marshal(kb.B)
	if len(is.db.uselessTagFiltersCache.Get(nil, kb.B)) > 0 {
		return nil, nil, errTooManyMetrics
	}
	metricIDs := &uint64set.Set{}
	if err := is.updateMetricIDsAll(metricIDs, maxMetrics); err != nil {
		return nil, nil, err
	}
	if metricIDs.Len() >= maxMetrics {
		kb.B = append(kb.B[:0], uselessNegativeTagFilterKeyPrefix)
		kb.B = encoding.MarshalUint64(kb.B, uint64(maxMetrics))
		kb.B = tfs.marshal(kb.B)
		is.db.uselessTagFiltersCache.Set(kb.B, uselessTagFilterCacheValue)
	}
	return nil, metricIDs, nil
}

func matchTagFilters(mn *MetricName, tfs []*tagFilter, kb *bytesutil.ByteBuffer) (bool, error) {
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)

	for i, tf := range tfs {
		if len(tf.key) == 0 {
			// Match against mn.MetricGroup.
			b := marshalTagValue(kb.B, nil)
			b = marshalTagValue(b, mn.MetricGroup)
			kb.B = b[:len(kb.B)]
			ok, err := matchTagFilter(b, tf)
			if err != nil {
				return false, fmt.Errorf("cannot match MetricGroup %q with tagFilter %s: %w", mn.MetricGroup, tf, err)
			}
			if !ok {
				// Move failed tf to start.
				// This should reduce the amount of useless work for the next mn.
				if i > 0 {
					tfs[0], tfs[i] = tfs[i], tfs[0]
				}
				return false, nil
			}
			continue
		}

		// Search for matching tag name.
		tagMatched := false
		tagSeen := false
		for j := range mn.Tags {
			tag := &mn.Tags[j]
			if string(tag.Key) != string(tf.key) {
				continue
			}

			// Found the matching tag name. Match the value.
			tagSeen = true
			b := tag.Marshal(kb.B)
			kb.B = b[:len(kb.B)]
			ok, err := matchTagFilter(b, tf)
			if err != nil {
				return false, fmt.Errorf("cannot match tag %q with tagFilter %s: %w", tag, tf, err)
			}
			if !ok {
				// Move failed tf to start.
				// This should reduce the amount of useless work for the next mn.
				if i > 0 {
					tfs[0], tfs[i] = tfs[i], tfs[0]
				}
				return false, nil
			}
			tagMatched = true
			break
		}
		if !tagSeen && tf.isNegative && !tf.isEmptyMatch {
			// tf contains negative filter for non-exsisting tag key
			// and this filter doesn't match empty string, i.e. {non_existing_tag_key!="foobar"}
			// Such filter matches anything.
			//
			// Note that the filter `{non_existing_tag_key!~"|foobar"}` shouldn't match anything,
			// since it is expected that it matches non-empty `non_existing_tag_key`.
			// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/546 for details.
			continue
		}
		if tagMatched {
			// tf matches mn. Go to the next tf.
			continue
		}
		// Matching tag name wasn't found.
		// Move failed tf to start.
		// This should reduce the amount of useless work for the next mn.
		if i > 0 {
			tfs[0], tfs[i] = tfs[i], tfs[0]
		}
		return false, nil
	}
	return true, nil
}

func matchTagFilter(b []byte, tf *tagFilter) (bool, error) {
	if !bytes.HasPrefix(b, tf.prefix) {
		return tf.isNegative, nil
	}
	ok, err := tf.matchSuffix(b[len(tf.prefix):])
	if err != nil {
		return false, err
	}
	if !ok {
		return tf.isNegative, nil
	}
	return !tf.isNegative, nil
}

func (is *indexSearch) searchMetricIDs(tfss []*TagFilters, tr TimeRange, maxMetrics int) ([]uint64, error) {
	metricIDs := &uint64set.Set{}
	for _, tfs := range tfss {
		if len(tfs.tfs) == 0 {
			// Return all the metric ids
			if err := is.updateMetricIDsAll(metricIDs, maxMetrics+1); err != nil {
				return nil, err
			}
			if metricIDs.Len() > maxMetrics {
				return nil, fmt.Errorf("the number of unique timeseries exceeds %d; either narrow down the search or increase -search.maxUniqueTimeseries", maxMetrics)
			}
			// Stop the iteration, since we cannot find more metric ids with the remaining tfss.
			break
		}
		if err := is.updateMetricIDsForTagFilters(metricIDs, tfs, tr, maxMetrics+1); err != nil {
			return nil, err
		}
		if metricIDs.Len() > maxMetrics {
			return nil, fmt.Errorf("the number of matching unique timeseries exceeds %d; either narrow down the search or increase -search.maxUniqueTimeseries", maxMetrics)
		}
	}
	if metricIDs.Len() == 0 {
		// Nothing found
		return nil, nil
	}

	sortedMetricIDs := metricIDs.AppendTo(nil)

	// Filter out deleted metricIDs.
	dmis := is.db.getDeletedMetricIDs()
	if dmis.Len() > 0 {
		metricIDsFiltered := sortedMetricIDs[:0]
		for _, metricID := range sortedMetricIDs {
			if !dmis.Has(metricID) {
				metricIDsFiltered = append(metricIDsFiltered, metricID)
			}
		}
		sortedMetricIDs = metricIDsFiltered
	}

	return sortedMetricIDs, nil
}

func (is *indexSearch) updateMetricIDsForTagFilters(metricIDs *uint64set.Set, tfs *TagFilters, tr TimeRange, maxMetrics int) error {
	err := is.tryUpdatingMetricIDsForDateRange(metricIDs, tfs, tr, maxMetrics)
	if err == nil {
		// Fast path: found metricIDs by date range.
		return nil
	}
	if err != errFallbackToMetricNameMatch {
		return err
	}

	// Slow path - try searching over the whole inverted index.

	// Sort tag filters for faster ts.Seek below.
	sort.Slice(tfs.tfs, func(i, j int) bool {
		return tfs.tfs[i].Less(&tfs.tfs[j])
	})
	minTf, minMetricIDs, err := is.getTagFilterWithMinMetricIDsCountOptimized(tfs, tr, maxMetrics)
	if err != nil {
		return err
	}

	// Find intersection of minTf with other tfs.
	var tfsPostponed []*tagFilter
	successfulIntersects := 0
	for i := range tfs.tfs {
		tf := &tfs.tfs[i]
		if tf == minTf {
			continue
		}
		mIDs, err := is.intersectMetricIDsWithTagFilter(tf, minMetricIDs)
		if err == errFallbackToMetricNameMatch {
			// The tag filter requires too many index scans. Postpone it,
			// so tag filters with lower number of index scans may be applied.
			tfsPostponed = append(tfsPostponed, tf)
			continue
		}
		if err != nil {
			return err
		}
		minMetricIDs = mIDs
		successfulIntersects++
	}
	if len(tfsPostponed) > 0 && successfulIntersects == 0 {
		return is.updateMetricIDsByMetricNameMatch(metricIDs, minMetricIDs, tfsPostponed)
	}
	for i, tf := range tfsPostponed {
		mIDs, err := is.intersectMetricIDsWithTagFilter(tf, minMetricIDs)
		if err == errFallbackToMetricNameMatch {
			return is.updateMetricIDsByMetricNameMatch(metricIDs, minMetricIDs, tfsPostponed[i:])
		}
		if err != nil {
			return err
		}
		minMetricIDs = mIDs
	}
	metricIDs.UnionMayOwn(minMetricIDs)
	return nil
}

const (
	uselessSingleTagFilterKeyPrefix   = 0
	uselessMultiTagFiltersKeyPrefix   = 1
	uselessNegativeTagFilterKeyPrefix = 2
	uselessTagIntersectKeyPrefix      = 3
)

var uselessTagFilterCacheValue = []byte("1")

func (is *indexSearch) getMetricIDsForTagFilter(tf *tagFilter, maxMetrics int) (*uint64set.Set, error) {
	if tf.isNegative {
		logger.Panicf("BUG: isNegative must be false")
	}
	metricIDs := &uint64set.Set{}
	if len(tf.orSuffixes) > 0 {
		// Fast path for orSuffixes - seek for rows for each value from orSuffxies.
		if err := is.updateMetricIDsForOrSuffixesNoFilter(tf, maxMetrics, metricIDs); err != nil {
			if err == errFallbackToMetricNameMatch {
				return nil, err
			}
			return nil, fmt.Errorf("error when searching for metricIDs for tagFilter in fast path: %w; tagFilter=%s", err, tf)
		}
		return metricIDs, nil
	}

	// Slow path - scan for all the rows with the given prefix.
	maxLoops := maxMetrics * maxIndexScanSlowLoopsPerMetric
	err := is.getMetricIDsForTagFilterSlow(tf, maxLoops, func(metricID uint64) bool {
		metricIDs.Add(metricID)
		return metricIDs.Len() < maxMetrics
	})
	if err != nil {
		if err == errFallbackToMetricNameMatch {
			return nil, err
		}
		return nil, fmt.Errorf("error when searching for metricIDs for tagFilter in slow path: %w; tagFilter=%s", err, tf)
	}
	return metricIDs, nil
}

func (is *indexSearch) getMetricIDsForTagFilterSlow(tf *tagFilter, maxLoops int, f func(metricID uint64) bool) error {
	if len(tf.orSuffixes) > 0 {
		logger.Panicf("BUG: the getMetricIDsForTagFilterSlow must be called only for empty tf.orSuffixes; got %s", tf.orSuffixes)
	}

	// Scan all the rows with tf.prefix and call f on every tf match.
	loops := 0
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	mp.Reset()
	var prevMatchingSuffix []byte
	var prevMatch bool
	prefix := tf.prefix
	ts.Seek(prefix)
	for ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			return nil
		}
		tail := item[len(prefix):]
		n := bytes.IndexByte(tail, tagSeparatorChar)
		if n < 0 {
			return fmt.Errorf("invalid tag->metricIDs line %q: cannot find tagSeparatorChar=%d", item, tagSeparatorChar)
		}
		suffix := tail[:n+1]
		tail = tail[n+1:]
		if err := mp.InitOnlyTail(item, tail); err != nil {
			return err
		}
		if prevMatch && string(suffix) == string(prevMatchingSuffix) {
			// Fast path: the same tag value found.
			// There is no need in checking it again with potentially
			// slow tf.matchSuffix, which may call regexp.
			loops += mp.MetricIDsLen()
			if loops > maxLoops {
				return errFallbackToMetricNameMatch
			}
			mp.ParseMetricIDs()
			for _, metricID := range mp.MetricIDs {
				if !f(metricID) {
					return nil
				}
			}
			continue
		}

		// Slow path: need tf.matchSuffix call.
		ok, err := tf.matchSuffix(suffix)
		if err != nil {
			return fmt.Errorf("error when matching %s against suffix %q: %w", tf, suffix, err)
		}
		if !ok {
			prevMatch = false
			if mp.MetricIDsLen() < maxMetricIDsPerRow/2 {
				// If the current row contains non-full metricIDs list,
				// then it is likely the next row contains the next tag value.
				// So skip seeking for the next tag value, since it will be slower than just ts.NextItem call.
				continue
			}
			// Optimization: skip all the metricIDs for the given tag value
			kb.B = append(kb.B[:0], item[:len(item)-len(tail)]...)
			// The last char in kb.B must be tagSeparatorChar. Just increment it
			// in order to jump to the next tag value.
			if len(kb.B) == 0 || kb.B[len(kb.B)-1] != tagSeparatorChar || tagSeparatorChar >= 0xff {
				return fmt.Errorf("data corruption: the last char in k=%X must be %X", kb.B, tagSeparatorChar)
			}
			kb.B[len(kb.B)-1]++
			ts.Seek(kb.B)
			continue
		}
		prevMatch = true
		prevMatchingSuffix = append(prevMatchingSuffix[:0], suffix...)
		loops += mp.MetricIDsLen()
		if loops > maxLoops {
			return errFallbackToMetricNameMatch
		}
		mp.ParseMetricIDs()
		for _, metricID := range mp.MetricIDs {
			if !f(metricID) {
				return nil
			}
		}
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for tag filter prefix %q: %w", prefix, err)
	}
	return nil
}

func (is *indexSearch) updateMetricIDsForOrSuffixesNoFilter(tf *tagFilter, maxMetrics int, metricIDs *uint64set.Set) error {
	if tf.isNegative {
		logger.Panicf("BUG: isNegative must be false")
	}
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	for _, orSuffix := range tf.orSuffixes {
		kb.B = append(kb.B[:0], tf.prefix...)
		kb.B = append(kb.B, orSuffix...)
		kb.B = append(kb.B, tagSeparatorChar)
		if err := is.updateMetricIDsForOrSuffixNoFilter(kb.B, maxMetrics, metricIDs); err != nil {
			return err
		}
		if metricIDs.Len() >= maxMetrics {
			return nil
		}
	}
	return nil
}

func (is *indexSearch) updateMetricIDsForOrSuffixesWithFilter(tf *tagFilter, metricIDs, filter *uint64set.Set) error {
	sortedFilter := filter.AppendTo(nil)
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	for _, orSuffix := range tf.orSuffixes {
		kb.B = append(kb.B[:0], tf.prefix...)
		kb.B = append(kb.B, orSuffix...)
		kb.B = append(kb.B, tagSeparatorChar)
		if err := is.updateMetricIDsForOrSuffixWithFilter(kb.B, metricIDs, sortedFilter, tf.isNegative); err != nil {
			return err
		}
	}
	return nil
}

func (is *indexSearch) updateMetricIDsForOrSuffixNoFilter(prefix []byte, maxMetrics int, metricIDs *uint64set.Set) error {
	ts := &is.ts
	mp := &is.mp
	mp.Reset()
	maxLoops := maxMetrics * maxIndexScanLoopsPerMetric
	loops := 0
	ts.Seek(prefix)
	for metricIDs.Len() < maxMetrics && ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			return nil
		}
		if err := mp.InitOnlyTail(item, item[len(prefix):]); err != nil {
			return err
		}
		loops += mp.MetricIDsLen()
		if loops > maxLoops {
			return errFallbackToMetricNameMatch
		}
		mp.ParseMetricIDs()
		metricIDs.AddMulti(mp.MetricIDs)
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for tag filter prefix %q: %w", prefix, err)
	}
	return nil
}

func (is *indexSearch) updateMetricIDsForOrSuffixWithFilter(prefix []byte, metricIDs *uint64set.Set, sortedFilter []uint64, isNegative bool) error {
	if len(sortedFilter) == 0 {
		return nil
	}
	firstFilterMetricID := sortedFilter[0]
	lastFilterMetricID := sortedFilter[len(sortedFilter)-1]
	ts := &is.ts
	mp := &is.mp
	mp.Reset()
	maxLoops := len(sortedFilter) * maxIndexScanLoopsPerMetric
	loops := 0
	ts.Seek(prefix)
	var sf []uint64
	var metricID uint64
	for ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			return nil
		}
		if err := mp.InitOnlyTail(item, item[len(prefix):]); err != nil {
			return err
		}
		firstMetricID, lastMetricID := mp.FirstAndLastMetricIDs()
		if lastMetricID < firstFilterMetricID {
			// Skip the item, since it contains metricIDs lower
			// than metricIDs in sortedFilter.
			continue
		}
		if firstMetricID > lastFilterMetricID {
			// Stop searching, since the current item and all the subsequent items
			// contain metricIDs higher than metricIDs in sortedFilter.
			return nil
		}
		sf = sortedFilter
		loops += mp.MetricIDsLen()
		if loops > maxLoops {
			return errFallbackToMetricNameMatch
		}
		mp.ParseMetricIDs()
		for _, metricID = range mp.MetricIDs {
			if len(sf) == 0 {
				break
			}
			if metricID > sf[0] {
				n := binarySearchUint64(sf, metricID)
				sf = sf[n:]
				if len(sf) == 0 {
					break
				}
			}
			if metricID < sf[0] {
				continue
			}
			if isNegative {
				metricIDs.Del(metricID)
			} else {
				metricIDs.Add(metricID)
			}
			sf = sf[1:]
		}
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for tag filter prefix %q: %w", prefix, err)
	}
	return nil
}

func binarySearchUint64(a []uint64, v uint64) uint {
	// Copy-pasted sort.Search from https://golang.org/src/sort/search.go?s=2246:2286#L49
	i, j := uint(0), uint(len(a))
	for i < j {
		h := (i + j) >> 1
		if h < uint(len(a)) && a[h] < v {
			i = h + 1
		} else {
			j = h
		}
	}
	return i
}

var errFallbackToMetricNameMatch = errors.New("fall back to updateMetricIDsByMetricNameMatch because of too many index scan loops")

var errMissingMetricIDsForDate = errors.New("missing metricIDs for date")

func (is *indexSearch) getMetricIDsForTimeRange(tr TimeRange, maxMetrics int) (*uint64set.Set, error) {
	atomic.AddUint64(&is.db.recentHourMetricIDsSearchCalls, 1)
	metricIDs, ok := is.getMetricIDsForRecentHours(tr, maxMetrics)
	if ok {
		// Fast path: tr covers the current and / or the previous hour.
		// Return the full list of metric ids for this time range.
		atomic.AddUint64(&is.db.recentHourMetricIDsSearchHits, 1)
		return metricIDs, nil
	}

	// Slow path: collect the metric ids for all the days covering the given tr.
	atomic.AddUint64(&is.db.dateMetricIDsSearchCalls, 1)
	minDate := uint64(tr.MinTimestamp) / msecPerDay
	maxDate := uint64(tr.MaxTimestamp) / msecPerDay
	if maxDate-minDate > maxDaysForDateMetricIDs {
		// Too much dates must be covered. Give up.
		return nil, errMissingMetricIDsForDate
	}
	if minDate == maxDate {
		// Fast path - query on a single day.
		metricIDs, err := is.getMetricIDsForDate(minDate, maxMetrics)
		if err != nil {
			return nil, err
		}
		atomic.AddUint64(&is.db.dateMetricIDsSearchHits, 1)
		return metricIDs, nil
	}

	// Slower path - query over multiple days in parallel.
	metricIDs = &uint64set.Set{}
	var wg sync.WaitGroup
	var errGlobal error
	var mu sync.Mutex // protects metricIDs + errGlobal from concurrent access below.
	for minDate <= maxDate {
		wg.Add(1)
		go func(date uint64) {
			defer wg.Done()
			isLocal := is.db.getIndexSearch()
			defer is.db.putIndexSearch(isLocal)
			m, err := isLocal.getMetricIDsForDate(date, maxMetrics)
			mu.Lock()
			defer mu.Unlock()
			if errGlobal != nil {
				return
			}
			if err != nil {
				errGlobal = err
				return
			}
			if metricIDs.Len() < maxMetrics {
				metricIDs.UnionMayOwn(m)
			}
		}(minDate)
		minDate++
	}
	wg.Wait()
	if errGlobal != nil {
		return nil, errGlobal
	}
	atomic.AddUint64(&is.db.dateMetricIDsSearchHits, 1)
	return metricIDs, nil
}

func (is *indexSearch) tryUpdatingMetricIDsForDateRange(metricIDs *uint64set.Set, tfs *TagFilters, tr TimeRange, maxMetrics int) error {
	atomic.AddUint64(&is.db.dateRangeSearchCalls, 1)
	minDate := uint64(tr.MinTimestamp) / msecPerDay
	maxDate := uint64(tr.MaxTimestamp) / msecPerDay
	if minDate < is.db.startDateForPerDayInvertedIndex || maxDate < minDate {
		// Per-day inverted index doesn't cover the selected date range.
		return errFallbackToMetricNameMatch
	}
	if maxDate-minDate > maxDaysForDateMetricIDs {
		// Too much dates must be covered. Give up, since it may be slow.
		return errFallbackToMetricNameMatch
	}
	if minDate == maxDate {
		// Fast path - query only a single date.
		m, err := is.getMetricIDsForDateAndFilters(minDate, tfs, maxMetrics)
		if err != nil {
			return err
		}
		metricIDs.UnionMayOwn(m)
		atomic.AddUint64(&is.db.dateRangeSearchHits, 1)
		return nil
	}

	// Slower path - search for metricIDs for each day in parallel.
	var wg sync.WaitGroup
	var errGlobal error
	var mu sync.Mutex // protects metricIDs + errGlobal vars from concurrent access below
	for minDate <= maxDate {
		wg.Add(1)
		go func(date uint64) {
			defer wg.Done()
			isLocal := is.db.getIndexSearch()
			defer is.db.putIndexSearch(isLocal)
			m, err := isLocal.getMetricIDsForDateAndFilters(date, tfs, maxMetrics)
			mu.Lock()
			defer mu.Unlock()
			if errGlobal != nil {
				return
			}
			if err != nil {
				if err == errFallbackToMetricNameMatch {
					// The per-date search is too expensive. Probably it is faster to perform global search
					// using metric name match.
					errGlobal = err
					return
				}
				dateStr := time.Unix(int64(date*24*3600), 0)
				errGlobal = fmt.Errorf("cannot search for metricIDs for %s: %w", dateStr, err)
				return
			}
			if metricIDs.Len() < maxMetrics {
				metricIDs.UnionMayOwn(m)
			}
		}(minDate)
		minDate++
	}
	wg.Wait()
	if errGlobal != nil {
		return errGlobal
	}
	atomic.AddUint64(&is.db.dateRangeSearchHits, 1)
	return nil
}

func (is *indexSearch) getMetricIDsForDateAndFilters(date uint64, tfs *TagFilters, maxMetrics int) (*uint64set.Set, error) {
	// Sort tfs by the number of matching filters from previous queries.
	// This way we limit the amount of work below by applying more specific filters at first.
	type tagFilterWithCount struct {
		tf    *tagFilter
		count uint64
	}
	tfsWithCount := make([]tagFilterWithCount, len(tfs.tfs))
	kb := &is.kb
	var buf []byte
	for i := range tfs.tfs {
		tf := &tfs.tfs[i]
		kb.B = appendDateTagFilterCacheKey(kb.B[:0], date, tf)
		buf = is.db.metricIDsPerDateTagFilterCache.Get(buf[:0], kb.B)
		count := uint64(0)
		if len(buf) == 8 {
			count = encoding.UnmarshalUint64(buf)
		}
		tfsWithCount[i] = tagFilterWithCount{
			tf:    tf,
			count: count,
		}
	}
	sort.Slice(tfsWithCount, func(i, j int) bool {
		a, b := &tfsWithCount[i], &tfsWithCount[j]
		if a.count != b.count {
			return a.count < b.count
		}
		return a.tf.Less(b.tf)
	})

	// Populate metricIDs with the first non-negative filter.
	var tfsPostponed []*tagFilter
	var metricIDs *uint64set.Set
	maxDateMetrics := maxMetrics * 50
	tfsRemainingWithCount := tfsWithCount[:0]
	for i := range tfsWithCount {
		tf := tfsWithCount[i].tf
		if tf.isNegative {
			tfsRemainingWithCount = append(tfsRemainingWithCount, tfsWithCount[i])
			continue
		}
		m, err := is.getMetricIDsForDateTagFilter(tf, date, tfs.commonPrefix, maxDateMetrics)
		if err != nil {
			return nil, err
		}
		if m.Len() >= maxDateMetrics {
			// Too many time series found by a single tag filter. Postpone applying this filter via metricName match.
			tfsPostponed = append(tfsPostponed, tf)
			continue
		}
		metricIDs = m
		i++
		for i < len(tfsWithCount) {
			tfsRemainingWithCount = append(tfsRemainingWithCount, tfsWithCount[i])
			i++
		}
		break
	}
	if metricIDs == nil {
		// All the filters in tfs are negative or match too many time series.
		// Populate all the metricIDs for the given (date),
		// so later they can be filtered out with negative filters.
		m, err := is.getMetricIDsForDate(date, maxDateMetrics)
		if err != nil {
			if err == errMissingMetricIDsForDate {
				// Zero time series were written on the given date.
				// It is OK, since (date, metricID) entries must exist for the given date
				// according to startDateForPerDayInvertedIndex.
				return nil, nil
			}
			return nil, fmt.Errorf("cannot obtain all the metricIDs: %w", err)
		}
		if m.Len() >= maxDateMetrics {
			// Too many time series found for the given (date). Fall back to global search.
			return nil, errFallbackToMetricNameMatch
		}
		metricIDs = m
	}

	// Intersect metricIDs with the rest of filters.
	for i := range tfsRemainingWithCount {
		tfWithCount := tfsRemainingWithCount[i]
		if n := uint64(metricIDs.Len()); n < 1000 || (n < tfWithCount.count/maxIndexScanLoopsPerMetric && n < uint64(maxMetrics)/10) {
			// It should be faster performing metricName match on the remaining filters
			// instead of scanning big number of entries in the inverted index for these filters.
			for i < len(tfsRemainingWithCount) {
				tfsPostponed = append(tfsPostponed, tfsRemainingWithCount[i].tf)
				i++
			}
			break
		}
		tf := tfWithCount.tf
		m, err := is.getMetricIDsForDateTagFilter(tf, date, tfs.commonPrefix, maxDateMetrics)
		if err != nil {
			return nil, err
		}
		if m.Len() >= maxDateMetrics {
			// Too many time series found by a single tag filter. Postpone applying this filter via metricName match.
			tfsPostponed = append(tfsPostponed, tf)
			continue
		}
		if tf.isNegative {
			metricIDs.Subtract(m)
		} else {
			metricIDs.Intersect(m)
		}
		if metricIDs.Len() == 0 {
			// Short circuit - there is no need in applying the remaining filters to empty set.
			return nil, nil
		}
	}
	if len(tfsPostponed) > 0 {
		if n := metricIDs.Len(); n > 50000 && n > maxMetrics/10 {
			// It will be slow to perform metricName match on this number of time series.
			// Fall back to global search.
			return nil, errFallbackToMetricNameMatch
		}
		// Apply the postponed filters via metricName match.
		var m uint64set.Set
		if err := is.updateMetricIDsByMetricNameMatch(&m, metricIDs, tfsPostponed); err != nil {
			return nil, err
		}
		return &m, nil
	}
	return metricIDs, nil
}

func (is *indexSearch) getMetricIDsForRecentHours(tr TimeRange, maxMetrics int) (*uint64set.Set, bool) {
	minHour := uint64(tr.MinTimestamp) / msecPerHour
	maxHour := uint64(tr.MaxTimestamp) / msecPerHour
	hmCurr := is.db.currHourMetricIDs.Load().(*hourMetricIDs)
	if maxHour == hmCurr.hour && minHour == maxHour && hmCurr.isFull {
		// The tr fits the current hour.
		// Return a copy of hmCurr.m, because the caller may modify
		// the returned map.
		if hmCurr.m.Len() > maxMetrics {
			return nil, false
		}
		return hmCurr.m.Clone(), true
	}
	hmPrev := is.db.prevHourMetricIDs.Load().(*hourMetricIDs)
	if maxHour == hmPrev.hour && minHour == maxHour && hmPrev.isFull {
		// The tr fits the previous hour.
		// Return a copy of hmPrev.m, because the caller may modify
		// the returned map.
		if hmPrev.m.Len() > maxMetrics {
			return nil, false
		}
		return hmPrev.m.Clone(), true
	}
	if maxHour == hmCurr.hour && minHour == hmPrev.hour && hmCurr.isFull && hmPrev.isFull {
		// The tr spans the previous and the current hours.
		if hmCurr.m.Len()+hmPrev.m.Len() > maxMetrics {
			return nil, false
		}
		metricIDs := hmCurr.m.Clone()
		metricIDs.Union(hmPrev.m)
		return metricIDs, true
	}
	return nil, false
}

func (is *indexSearch) storeDateMetricID(date, metricID uint64) error {
	items := getIndexItems()
	defer putIndexItems(items)

	items.B = marshalCommonPrefix(items.B, nsPrefixDateToMetricID)
	items.B = encoding.MarshalUint64(items.B, date)
	items.B = encoding.MarshalUint64(items.B, metricID)
	items.Next()

	// Create per-day inverted index entries for metricID.
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	mn := GetMetricName()
	defer PutMetricName(mn)
	var err error
	// There is no need in searching for metric name in is.db.extDB,
	// Since the storeDateMetricID function is called only after the metricID->metricName
	// is added into the current is.db.
	kb.B, err = is.searchMetricName(kb.B[:0], metricID)
	if err != nil {
		if err == io.EOF {
			logger.Errorf("missing metricName by metricID %d; this could be the case after unclean shutdown; "+
				"deleting the metricID, so it could be re-created next time", metricID)
			if err := is.db.deleteMetricIDs([]uint64{metricID}); err != nil {
				return fmt.Errorf("cannot delete metricID %d after unclean shutdown: %w", metricID, err)
			}
			return nil
		}
		return fmt.Errorf("cannot find metricName by metricID %d: %w", metricID, err)
	}
	if err = mn.Unmarshal(kb.B); err != nil {
		return fmt.Errorf("cannot unmarshal metricName %q obtained by metricID %d: %w", metricID, kb.B, err)
	}
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
	kb.B = encoding.MarshalUint64(kb.B, date)

	items.B = append(items.B, kb.B...)
	items.B = marshalTagValue(items.B, nil)
	items.B = marshalTagValue(items.B, mn.MetricGroup)
	items.B = encoding.MarshalUint64(items.B, metricID)
	items.Next()
	addReverseMetricGroupIfNeeded(items, kb.B, mn, metricID)
	for i := range mn.Tags {
		tag := &mn.Tags[i]
		items.B = append(items.B, kb.B...)
		items.B = tag.Marshal(items.B)
		items.B = encoding.MarshalUint64(items.B, metricID)
		items.Next()
	}
	if err = is.db.tb.AddItems(items.Items); err != nil {
		return fmt.Errorf("cannot add per-day entires for metricID %d: %w", metricID, err)
	}
	return nil
}

func addReverseMetricGroupIfNeeded(items *indexItems, prefix []byte, mn *MetricName, metricID uint64) {
	if bytes.IndexByte(mn.MetricGroup, '.') < 0 {
		// The reverse metric group is needed only for Graphite-like metrics with points.
		return
	}
	// This is most likely a Graphite metric like 'foo.bar.baz'.
	// Store reverse metric name 'zab.rab.oof' in order to speed up search for '*.bar.baz'
	// when the Graphite wildcard has a suffix matching small number of time series.
	items.B = append(items.B, prefix...)
	items.B = marshalTagValue(items.B, graphiteReverseTagKey)
	revBuf := kbPool.Get()
	revBuf.B = reverseBytes(revBuf.B[:0], mn.MetricGroup)
	items.B = marshalTagValue(items.B, revBuf.B)
	kbPool.Put(revBuf)
	items.B = encoding.MarshalUint64(items.B, metricID)
	items.Next()
}

// The tag key for reverse metric name used for speeding up searching
// for Graphite wildcards with suffix matching small number of time series,
// i.e. '*.bar.baz'.
//
// It is expected that the given key isn't be used by users.
var graphiteReverseTagKey = []byte("\xff")

func reverseBytes(dst, src []byte) []byte {
	for i := len(src) - 1; i >= 0; i-- {
		dst = append(dst, src[i])
	}
	return dst
}

func (is *indexSearch) hasDateMetricID(date, metricID uint64) (bool, error) {
	ts := &is.ts
	kb := &is.kb
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixDateToMetricID)
	kb.B = encoding.MarshalUint64(kb.B, date)
	kb.B = encoding.MarshalUint64(kb.B, metricID)
	if err := ts.FirstItemWithPrefix(kb.B); err != nil {
		if err == io.EOF {
			return false, nil
		}
		return false, fmt.Errorf("error when searching for (date=%d, metricID=%d) entry: %w", date, metricID, err)
	}
	if string(ts.Item) != string(kb.B) {
		return false, fmt.Errorf("unexpected entry for (date=%d, metricID=%d); got %q; want %q", date, metricID, ts.Item, kb.B)
	}
	return true, nil
}

func (is *indexSearch) getMetricIDsForDateTagFilter(tf *tagFilter, date uint64, commonPrefix []byte, maxMetrics int) (*uint64set.Set, error) {
	// Augument tag filter prefix for per-date search instead of global search.
	if !bytes.HasPrefix(tf.prefix, commonPrefix) {
		logger.Panicf("BUG: unexpected tf.prefix %q; must start with commonPrefix %q", tf.prefix, commonPrefix)
	}
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
	kb.B = encoding.MarshalUint64(kb.B, date)
	kb.B = append(kb.B, tf.prefix[len(commonPrefix):]...)

	tfNew := *tf
	tfNew.isNegative = false // isNegative for the original tf is handled by the caller.
	tfNew.prefix = kb.B
	metricIDs, err := is.getMetricIDsForTagFilter(&tfNew, maxMetrics)

	// Store the number of matching metricIDs in the cache in order to sort tag filters
	// in ascending number of matching metricIDs on the next search.
	is.kb.B = appendDateTagFilterCacheKey(is.kb.B[:0], date, tf)
	metricIDsLen := uint64(metricIDs.Len())
	if err != nil {
		// Set metricIDsLen to maxMetrics, so the given entry will be moved to the end
		// of tag filters on the next search.
		metricIDsLen = uint64(maxMetrics)
	}
	kb.B = encoding.MarshalUint64(kb.B[:0], metricIDsLen)
	is.db.metricIDsPerDateTagFilterCache.Set(is.kb.B, kb.B)
	return metricIDs, err
}

func appendDateTagFilterCacheKey(dst []byte, date uint64, tf *tagFilter) []byte {
	dst = encoding.MarshalUint64(dst, date)
	dst = tf.Marshal(dst)
	return dst
}

func (is *indexSearch) getMetricIDsForDate(date uint64, maxMetrics int) (*uint64set.Set, error) {
	// Extract all the metricIDs from (date, __name__=value)->metricIDs entries.
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
	kb.B = encoding.MarshalUint64(kb.B, date)
	kb.B = marshalTagValue(kb.B, nil)
	var metricIDs uint64set.Set
	if err := is.updateMetricIDsForPrefix(kb.B, &metricIDs, maxMetrics); err != nil {
		return nil, err
	}
	if metricIDs.Len() == 0 {
		// There are no metricIDs for the given date.
		// This may be the case for old data where (data, __name__=value)->metricIDs entries weren't available.
		return nil, errMissingMetricIDsForDate
	}
	return &metricIDs, nil
}

func (is *indexSearch) updateMetricIDsAll(metricIDs *uint64set.Set, maxMetrics int) error {
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	// Extract all the metricIDs from (__name__=value)->metricIDs entries.
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	kb.B = marshalTagValue(kb.B, nil)
	return is.updateMetricIDsForPrefix(kb.B, metricIDs, maxMetrics)
}

func (is *indexSearch) updateMetricIDsForPrefix(prefix []byte, metricIDs *uint64set.Set, maxMetrics int) error {
	ts := &is.ts
	mp := &is.mp
	ts.Seek(prefix)
	for ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			return nil
		}
		tail := item[len(prefix):]
		n := bytes.IndexByte(tail, tagSeparatorChar)
		if n < 0 {
			return fmt.Errorf("invalid tag->metricIDs line %q: cannot find tagSeparatorChar %d", item, tagSeparatorChar)
		}
		tail = tail[n+1:]
		if err := mp.InitOnlyTail(item, tail); err != nil {
			return err
		}
		mp.ParseMetricIDs()
		metricIDs.AddMulti(mp.MetricIDs)
		if metricIDs.Len() >= maxMetrics {
			return nil
		}
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for all metricIDs by prefix %q: %w", prefix, err)
	}
	return nil
}

// The maximum number of index scan loops.
// Bigger number of loops is slower than updateMetricIDsByMetricNameMatch
// over the found metrics.
const maxIndexScanLoopsPerMetric = 100

// The maximum number of slow index scan loops.
// Bigger number of loops is slower than updateMetricIDsByMetricNameMatch
// over the found metrics.
const maxIndexScanSlowLoopsPerMetric = 20

func (is *indexSearch) intersectMetricIDsWithTagFilter(tf *tagFilter, filter *uint64set.Set) (*uint64set.Set, error) {
	if filter.Len() == 0 {
		return nil, nil
	}
	kb := &is.kb
	filterLenRounded := (uint64(filter.Len()) / 1024) * 1024
	kb.B = append(kb.B[:0], uselessTagIntersectKeyPrefix)
	kb.B = encoding.MarshalUint64(kb.B, filterLenRounded)
	kb.B = tf.Marshal(kb.B)
	if len(is.db.uselessTagFiltersCache.Get(nil, kb.B)) > 0 {
		// Skip useless work, since the intersection will return
		// errFallbackToMetricNameMatc for the given filter.
		return nil, errFallbackToMetricNameMatch
	}
	metricIDs, err := is.intersectMetricIDsWithTagFilterNocache(tf, filter)
	if err == nil {
		return metricIDs, err
	}
	if err != errFallbackToMetricNameMatch {
		return nil, err
	}
	kb.B = append(kb.B[:0], uselessTagIntersectKeyPrefix)
	kb.B = encoding.MarshalUint64(kb.B, filterLenRounded)
	kb.B = tf.Marshal(kb.B)
	is.db.uselessTagFiltersCache.Set(kb.B, uselessTagFilterCacheValue)
	return nil, errFallbackToMetricNameMatch
}

func (is *indexSearch) intersectMetricIDsWithTagFilterNocache(tf *tagFilter, filter *uint64set.Set) (*uint64set.Set, error) {
	metricIDs := filter
	if !tf.isNegative {
		metricIDs = &uint64set.Set{}
	}
	if len(tf.orSuffixes) > 0 {
		// Fast path for orSuffixes - seek for rows for each value from orSuffixes.
		if err := is.updateMetricIDsForOrSuffixesWithFilter(tf, metricIDs, filter); err != nil {
			if err == errFallbackToMetricNameMatch {
				return nil, err
			}
			return nil, fmt.Errorf("error when intersecting metricIDs for tagFilter in fast path: %w; tagFilter=%s", err, tf)
		}
		return metricIDs, nil
	}

	// Slow path - scan for all the rows with the given prefix.
	maxLoops := filter.Len() * maxIndexScanSlowLoopsPerMetric
	err := is.getMetricIDsForTagFilterSlow(tf, maxLoops, func(metricID uint64) bool {
		if tf.isNegative {
			// filter must be equal to metricIDs
			metricIDs.Del(metricID)
			return true
		}
		if filter.Has(metricID) {
			metricIDs.Add(metricID)
		}
		return true
	})
	if err != nil {
		if err == errFallbackToMetricNameMatch {
			return nil, err
		}
		return nil, fmt.Errorf("error when intersecting metricIDs for tagFilter in slow path: %w; tagFilter=%s", err, tf)
	}
	return metricIDs, nil
}

var kbPool bytesutil.ByteBufferPool

// Returns local unique MetricID.
func generateUniqueMetricID() uint64 {
	// It is expected that metricIDs returned from this function must be dense.
	// If they will be sparse, then this may hurt metric_ids intersection
	// performance with uint64set.Set.
	return atomic.AddUint64(&nextUniqueMetricID, 1)
}

// This number mustn't go backwards on restarts, otherwise metricID
// collisions are possible. So don't change time on the server
// between VictoriaMetrics restarts.
var nextUniqueMetricID = uint64(time.Now().UnixNano())

func marshalCommonPrefix(dst []byte, nsPrefix byte) []byte {
	dst = append(dst, nsPrefix)
	return dst
}

func unmarshalCommonPrefix(src []byte) ([]byte, byte, error) {
	if len(src) < commonPrefixLen {
		return nil, 0, fmt.Errorf("cannot unmarshal common prefix from %d bytes; need at least %d bytes; data=%X", len(src), commonPrefixLen, src)
	}
	prefix := src[0]
	return src[commonPrefixLen:], prefix, nil
}

// 1 byte for prefix
const commonPrefixLen = 1

type tagToMetricIDsRowParser struct {
	// NSPrefix contains the first byte parsed from the row after Init call.
	// This is either nsPrefixTagToMetricIDs or nsPrefixDateTagToMetricIDs.
	NSPrefix byte

	// Date contains parsed date for nsPrefixDateTagToMetricIDs rows after Init call
	Date uint64

	// MetricIDs contains parsed MetricIDs after ParseMetricIDs call
	MetricIDs []uint64

	// Tag contains parsed tag after Init call
	Tag Tag

	// tail contains the remaining unparsed metricIDs
	tail []byte
}

func (mp *tagToMetricIDsRowParser) Reset() {
	mp.NSPrefix = 0
	mp.Date = 0
	mp.MetricIDs = mp.MetricIDs[:0]
	mp.Tag.Reset()
	mp.tail = nil
}

// Init initializes mp from b, which should contain encoded tag->metricIDs row.
//
// b cannot be re-used until Reset call.
func (mp *tagToMetricIDsRowParser) Init(b []byte, nsPrefixExpected byte) error {
	tail, nsPrefix, err := unmarshalCommonPrefix(b)
	if err != nil {
		return fmt.Errorf("invalid tag->metricIDs row %q: %w", b, err)
	}
	if nsPrefix != nsPrefixExpected {
		return fmt.Errorf("invalid prefix for tag->metricIDs row %q; got %d; want %d", b, nsPrefix, nsPrefixExpected)
	}
	if nsPrefix == nsPrefixDateTagToMetricIDs {
		// unmarshal date.
		if len(tail) < 8 {
			return fmt.Errorf("cannot unmarshal date from (date, tag)->metricIDs row %q from %d bytes; want at least 8 bytes", b, len(tail))
		}
		mp.Date = encoding.UnmarshalUint64(tail)
		tail = tail[8:]
	}
	mp.NSPrefix = nsPrefix
	tail, err = mp.Tag.Unmarshal(tail)
	if err != nil {
		return fmt.Errorf("cannot unmarshal tag from tag->metricIDs row %q: %w", b, err)
	}
	return mp.InitOnlyTail(b, tail)
}

// MarshalPrefix marshals row prefix without tail to dst.
func (mp *tagToMetricIDsRowParser) MarshalPrefix(dst []byte) []byte {
	dst = marshalCommonPrefix(dst, mp.NSPrefix)
	if mp.NSPrefix == nsPrefixDateTagToMetricIDs {
		dst = encoding.MarshalUint64(dst, mp.Date)
	}
	dst = mp.Tag.Marshal(dst)
	return dst
}

// InitOnlyTail initializes mp.tail from tail.
//
// b must contain tag->metricIDs row.
// b cannot be re-used until Reset call.
func (mp *tagToMetricIDsRowParser) InitOnlyTail(b, tail []byte) error {
	if len(tail) == 0 {
		return fmt.Errorf("missing metricID in the tag->metricIDs row %q", b)
	}
	if len(tail)%8 != 0 {
		return fmt.Errorf("invalid tail length in the tag->metricIDs row; got %d bytes; must be multiple of 8 bytes", len(tail))
	}
	mp.tail = tail
	return nil
}

// EqualPrefix returns true if prefixes for mp and x are equal.
//
// Prefix contains (tag)
func (mp *tagToMetricIDsRowParser) EqualPrefix(x *tagToMetricIDsRowParser) bool {
	if !mp.Tag.Equal(&x.Tag) {
		return false
	}
	return mp.Date == x.Date && mp.NSPrefix == x.NSPrefix
}

// FirstAndLastMetricIDs returns the first and the last metricIDs in the mp.tail.
func (mp *tagToMetricIDsRowParser) FirstAndLastMetricIDs() (uint64, uint64) {
	tail := mp.tail
	if len(tail) < 8 {
		logger.Panicf("BUG: cannot unmarshal metricID from %d bytes; need 8 bytes", len(tail))
		return 0, 0
	}
	firstMetricID := encoding.UnmarshalUint64(tail)
	lastMetricID := firstMetricID
	if len(tail) > 8 {
		lastMetricID = encoding.UnmarshalUint64(tail[len(tail)-8:])
	}
	return firstMetricID, lastMetricID
}

// MetricIDsLen returns the number of MetricIDs in the mp.tail
func (mp *tagToMetricIDsRowParser) MetricIDsLen() int {
	return len(mp.tail) / 8
}

// ParseMetricIDs parses MetricIDs from mp.tail into mp.MetricIDs.
func (mp *tagToMetricIDsRowParser) ParseMetricIDs() {
	tail := mp.tail
	mp.MetricIDs = mp.MetricIDs[:0]
	n := len(tail) / 8
	if n <= cap(mp.MetricIDs) {
		mp.MetricIDs = mp.MetricIDs[:n]
	} else {
		mp.MetricIDs = append(mp.MetricIDs[:cap(mp.MetricIDs)], make([]uint64, n-cap(mp.MetricIDs))...)
	}
	metricIDs := mp.MetricIDs
	_ = metricIDs[n-1]
	for i := 0; i < n; i++ {
		if len(tail) < 8 {
			logger.Panicf("BUG: tail cannot be smaller than 8 bytes; got %d bytes; tail=%X", len(tail), tail)
			return
		}
		metricID := encoding.UnmarshalUint64(tail)
		metricIDs[i] = metricID
		tail = tail[8:]
	}
}

// IsDeletedTag verifies whether the tag from mp is deleted according to dmis.
//
// dmis must contain deleted MetricIDs.
func (mp *tagToMetricIDsRowParser) IsDeletedTag(dmis *uint64set.Set) bool {
	if dmis.Len() == 0 {
		return false
	}
	mp.ParseMetricIDs()
	for _, metricID := range mp.MetricIDs {
		if !dmis.Has(metricID) {
			return false
		}
	}
	return true
}

func mergeTagToMetricIDsRows(data []byte, items [][]byte) ([]byte, [][]byte) {
	data, items = mergeTagToMetricIDsRowsInternal(data, items, nsPrefixTagToMetricIDs)
	data, items = mergeTagToMetricIDsRowsInternal(data, items, nsPrefixDateTagToMetricIDs)
	return data, items
}

func mergeTagToMetricIDsRowsInternal(data []byte, items [][]byte, nsPrefix byte) ([]byte, [][]byte) {
	// Perform quick checks whether items contain rows starting from nsPrefix
	// based on the fact that items are sorted.
	if len(items) <= 2 {
		// The first and the last row must remain unchanged.
		return data, items
	}
	firstItem := items[0]
	if len(firstItem) > 0 && firstItem[0] > nsPrefix {
		return data, items
	}
	lastItem := items[len(items)-1]
	if len(lastItem) > 0 && lastItem[0] < nsPrefix {
		return data, items
	}

	// items contain at least one row starting from nsPrefix. Merge rows with common tag.
	tmm := getTagToMetricIDsRowsMerger()
	tmm.dataCopy = append(tmm.dataCopy[:0], data...)
	tmm.itemsCopy = append(tmm.itemsCopy[:0], items...)
	mp := &tmm.mp
	mpPrev := &tmm.mpPrev
	dstData := data[:0]
	dstItems := items[:0]
	for i, item := range items {
		if len(item) == 0 || item[0] != nsPrefix || i == 0 || i == len(items)-1 {
			// Write rows not starting with nsPrefix as-is.
			// Additionally write the first and the last row as-is in order to preserve
			// sort order for adjancent blocks.
			dstData, dstItems = tmm.flushPendingMetricIDs(dstData, dstItems, mpPrev)
			dstData = append(dstData, item...)
			dstItems = append(dstItems, dstData[len(dstData)-len(item):])
			continue
		}
		if err := mp.Init(item, nsPrefix); err != nil {
			logger.Panicf("FATAL: cannot parse row starting with nsPrefix %d during merge: %s", nsPrefix, err)
		}
		if mp.MetricIDsLen() >= maxMetricIDsPerRow {
			dstData, dstItems = tmm.flushPendingMetricIDs(dstData, dstItems, mpPrev)
			dstData = append(dstData, item...)
			dstItems = append(dstItems, dstData[len(dstData)-len(item):])
			continue
		}
		if !mp.EqualPrefix(mpPrev) {
			dstData, dstItems = tmm.flushPendingMetricIDs(dstData, dstItems, mpPrev)
		}
		mp.ParseMetricIDs()
		tmm.pendingMetricIDs = append(tmm.pendingMetricIDs, mp.MetricIDs...)
		mpPrev, mp = mp, mpPrev
		if len(tmm.pendingMetricIDs) >= maxMetricIDsPerRow {
			dstData, dstItems = tmm.flushPendingMetricIDs(dstData, dstItems, mpPrev)
		}
	}
	if len(tmm.pendingMetricIDs) > 0 {
		logger.Panicf("BUG: tmm.pendingMetricIDs must be empty at this point; got %d items: %d", len(tmm.pendingMetricIDs), tmm.pendingMetricIDs)
	}
	if !checkItemsSorted(dstItems) {
		// Items could become unsorted if initial items contain duplicate metricIDs:
		//
		//   item1: 1, 1, 5
		//   item2: 1, 4
		//
		// Items could become the following after the merge:
		//
		//   item1: 1, 5
		//   item2: 1, 4
		//
		// i.e. item1 > item2
		//
		// Leave the original items unmerged, so they can be merged next time.
		// This case should be quite rare - if multiple data points are simultaneously inserted
		// into the same new time series from multiple concurrent goroutines.
		atomic.AddUint64(&indexBlocksWithMetricIDsIncorrectOrder, 1)
		dstData = append(dstData[:0], tmm.dataCopy...)
		dstItems = dstItems[:0]
		// tmm.itemsCopy can point to overwritten data, so it must be updated
		// to point to real data from tmm.dataCopy.
		buf := dstData
		for _, item := range tmm.itemsCopy {
			dstItems = append(dstItems, buf[:len(item)])
			buf = buf[len(item):]
		}
		if !checkItemsSorted(dstItems) {
			logger.Panicf("BUG: the original items weren't sorted; items=%q", dstItems)
		}
	}
	putTagToMetricIDsRowsMerger(tmm)
	atomic.AddUint64(&indexBlocksWithMetricIDsProcessed, 1)
	return dstData, dstItems
}

var indexBlocksWithMetricIDsIncorrectOrder uint64
var indexBlocksWithMetricIDsProcessed uint64

func checkItemsSorted(items [][]byte) bool {
	if len(items) == 0 {
		return true
	}
	prevItem := items[0]
	for _, currItem := range items[1:] {
		if string(prevItem) > string(currItem) {
			return false
		}
		prevItem = currItem
	}
	return true
}

// maxMetricIDsPerRow limits the number of metricIDs in tag->metricIDs row.
//
// This reduces overhead on index and metaindex in lib/mergeset.
const maxMetricIDsPerRow = 64

type uint64Sorter []uint64

func (s uint64Sorter) Len() int { return len(s) }
func (s uint64Sorter) Less(i, j int) bool {
	return s[i] < s[j]
}
func (s uint64Sorter) Swap(i, j int) {
	s[i], s[j] = s[j], s[i]
}

type tagToMetricIDsRowsMerger struct {
	pendingMetricIDs uint64Sorter
	mp               tagToMetricIDsRowParser
	mpPrev           tagToMetricIDsRowParser

	itemsCopy [][]byte
	dataCopy  []byte
}

func (tmm *tagToMetricIDsRowsMerger) Reset() {
	tmm.pendingMetricIDs = tmm.pendingMetricIDs[:0]
	tmm.mp.Reset()
	tmm.mpPrev.Reset()

	tmm.itemsCopy = tmm.itemsCopy[:0]
	tmm.dataCopy = tmm.dataCopy[:0]
}

func (tmm *tagToMetricIDsRowsMerger) flushPendingMetricIDs(dstData []byte, dstItems [][]byte, mp *tagToMetricIDsRowParser) ([]byte, [][]byte) {
	if len(tmm.pendingMetricIDs) == 0 {
		// Nothing to flush
		return dstData, dstItems
	}
	// Use sort.Sort instead of sort.Slice in order to reduce memory allocations.
	sort.Sort(&tmm.pendingMetricIDs)
	tmm.pendingMetricIDs = removeDuplicateMetricIDs(tmm.pendingMetricIDs)

	// Marshal pendingMetricIDs
	dstDataLen := len(dstData)
	dstData = mp.MarshalPrefix(dstData)
	for _, metricID := range tmm.pendingMetricIDs {
		dstData = encoding.MarshalUint64(dstData, metricID)
	}
	dstItems = append(dstItems, dstData[dstDataLen:])
	tmm.pendingMetricIDs = tmm.pendingMetricIDs[:0]
	return dstData, dstItems
}

func removeDuplicateMetricIDs(sortedMetricIDs []uint64) []uint64 {
	if len(sortedMetricIDs) < 2 {
		return sortedMetricIDs
	}
	prevMetricID := sortedMetricIDs[0]
	hasDuplicates := false
	for _, metricID := range sortedMetricIDs[1:] {
		if prevMetricID == metricID {
			hasDuplicates = true
			break
		}
		prevMetricID = metricID
	}
	if !hasDuplicates {
		return sortedMetricIDs
	}
	dstMetricIDs := sortedMetricIDs[:1]
	prevMetricID = sortedMetricIDs[0]
	for _, metricID := range sortedMetricIDs[1:] {
		if prevMetricID == metricID {
			continue
		}
		dstMetricIDs = append(dstMetricIDs, metricID)
		prevMetricID = metricID
	}
	return dstMetricIDs
}

func getTagToMetricIDsRowsMerger() *tagToMetricIDsRowsMerger {
	v := tmmPool.Get()
	if v == nil {
		return &tagToMetricIDsRowsMerger{}
	}
	return v.(*tagToMetricIDsRowsMerger)
}

func putTagToMetricIDsRowsMerger(tmm *tagToMetricIDsRowsMerger) {
	tmm.Reset()
	tmmPool.Put(tmm)
}

var tmmPool sync.Pool