package storage

import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"os"
	"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/logger"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/memory"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/mergeset"
	"github.com/VictoriaMetrics/fastcache"
	xxhash "github.com/cespare/xxhash/v2"
)

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

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

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

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

	// Prefix for deleted MetricID entries.
	nsPrefixDeteletedMetricID = 4

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

// indexDB represents an index db.
type indexDB struct {
	name     string
	refCount uint64
	tb       *mergeset.Table

	extDB     *indexDB
	extDBLock sync.Mutex

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

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

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

	tagCachePrefixes     map[accountProjectKey]uint64
	tagCachePrefixesLock sync.RWMutex

	indexSearchPool sync.Pool

	// An inmemory map[uint64]struct{} of deleted metricIDs.
	//
	// The map holds deleted metricIDs for the current db and for the extDB.
	//
	// It is safe to keep the map in memory even for big number of deleted
	// metricIDs, since it occupies only 8 bytes per deleted metricID.
	deletedMetricIDs           atomic.Value
	deletedMetricIDsUpdateLock sync.Mutex

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

	mustDrop uint64
}

// accountProjectKey is used for maps keyed by (AccountID, ProjectID).
type accountProjectKey struct {
	AccountID uint32
	ProjectID uint32
}

// openIndexDB opens index db from the given path with the given caches.
func openIndexDB(path string, metricIDCache, metricNameCache *fastcache.Cache) (*indexDB, error) {
	tb, err := mergeset.OpenTable(path)
	if err != nil {
		return nil, fmt.Errorf("cannot open indexDB %q: %s", path, err)
	}

	name := filepath.Base(path)

	// Do not persist tagCache in files, since it is very volatile.
	mem := memory.Allowed()
	tagCache := fastcache.New(mem / 32)

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

		tagCache:        tagCache,
		metricIDCache:   metricIDCache,
		metricNameCache: metricNameCache,

		tagCachePrefixes: make(map[accountProjectKey]uint64),
	}

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

	return db, nil
}

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

	DeletedMetricsCount uint64

	IndexDBRefCount uint64

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

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

	m.IndexDBRefCount += atomic.LoadUint64(&db.refCount)
	m.MissingTSIDsForMetricID += atomic.LoadUint64(&db.missingTSIDsForMetricID)

	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()
		metricIDs := getSortedMetricIDs(dmisExt)
		db.updateDeletedMetricIDs(metricIDs)
	}

	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 n < 0 {
		logger.Panicf("BUG: negative refCount: %d", n)
	}
	if n > 0 {
		return
	}

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

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

	logger.Infof("dropping indexDB %q", tbPath)
	if err := os.RemoveAll(tbPath); err != nil {
		logger.Panicf("FATAL: cannot remove %q: %s", tbPath, err)
	}
	logger.Infof("indexDB %q has been dropped", tbPath)
}

func (db *indexDB) getFromTagCache(key []byte) ([]TSID, bool) {
	value := db.tagCache.GetBig(nil, key)
	if len(value) == 0 {
		return nil, false
	}
	tsids, err := unmarshalTSIDs(nil, value)
	if err != nil {
		logger.Panicf("FATAL: cannot unmarshal tsids from tagCache: %s", err)
	}
	return tsids, true
}

func (db *indexDB) putToTagCache(tsids []TSID, key []byte) {
	value := marshalTSIDs(nil, tsids)
	db.tagCache.SetBig(key, value)
}

func (db *indexDB) getFromMetricIDCache(dst *TSID, metricID uint64) error {
	// There is no need in prefixing the key with (accountID, projectID),
	// since metricID is globally unique across all (accountID, projectID) values.
	// See getUniqueUint64.

	// 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 prefixing the key with (accountID, projectID),
	// since metricID is globally unique across all (accountID, projectID) values.
	// See getUniqueUint64.

	// 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 (db *indexDB) marshalTagFiltersKey(dst []byte, tfss []*TagFilters) []byte {
	if len(tfss) == 0 {
		return nil
	}
	k := accountProjectKey{
		AccountID: tfss[0].accountID,
		ProjectID: tfss[0].projectID,
	}
	db.tagCachePrefixesLock.RLock()
	prefix := db.tagCachePrefixes[k]
	db.tagCachePrefixesLock.RUnlock()
	if prefix == 0 {
		// Create missing prefix.
		// It is if multiple concurrent goroutines call invalidateTagCache
		// for the same (accountID, projectID).
		prefix = db.invalidateTagCache(k.AccountID, k.ProjectID)
	}
	dst = encoding.MarshalUint64(dst, prefix)
	for _, tfs := range tfss {
		dst = append(dst, 0) // separator between tfs groups.
		for i := range tfs.tfs {
			dst = tfs.tfs[i].MarshalNoAccountIDProjectID(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: %s", 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 (db *indexDB) invalidateTagCache(accountID, projectID uint32) uint64 {
	// This function must be fast, since it is called each
	// time new timeseries is added.
	prefix := atomic.AddUint64(&tagCacheKeyPrefix, 1)
	k := accountProjectKey{
		AccountID: accountID,
		ProjectID: projectID,
	}
	db.tagCachePrefixesLock.Lock()
	db.tagCachePrefixes[k] = prefix
	db.tagCachePrefixesLock.Unlock()
	return prefix
}

var tagCacheKeyPrefix 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: %s", 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

	// 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: %s", 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: %s", 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)
	return is
}

func (db *indexDB) putIndexSearch(is *indexSearch) {
	is.ts.MustClose()
	is.kb.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: %s", metricName, err)
	}

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

	// Invalidate tag cache for the given (AccountID, ProjectID), since
	// it doesn't contain tags for the created mn -> TSID mapping.
	_ = db.invalidateTagCache(mn.AccountID, mn.ProjectID)

	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: %s", err)
		}
	}

	// The TSID wan't found in the external storage.
	// Generate it locally.
	dst.AccountID = mn.AccountID
	dst.ProjectID = mn.ProjectID
	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 = getUniqueUint64()
	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, mn.AccountID, mn.ProjectID)
	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, mn.AccountID, mn.ProjectID)
	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], nsPrefixTagToMetricID, mn.AccountID, mn.ProjectID)

	// 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()

	// 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 for the given accountID, projectID.
func (db *indexDB) SearchTagKeys(accountID, projectID uint32, maxTagKeys int) ([]string, error) {
	// TODO: cache results?

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

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

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

	keys := make([]string, 0, len(tks))
	for key := range tks {
		keys = append(keys, key)
	}

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

func (is *indexSearch) searchTagKeys(accountID, projectID uint32, tks map[string]struct{}, maxTagKeys int) error {
	ts := &is.ts
	kb := &is.kb
	dmis := is.db.getDeletedMetricIDs()
	commonPrefix := marshalCommonPrefix(nil, nsPrefixTagToMetricID, accountID, projectID)
	ts.Seek(commonPrefix)
	for len(tks) < maxTagKeys && ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, commonPrefix) {
			break
		}
		tail := item[len(commonPrefix):]

		// Unmarshal tag key into kb.B
		var err error
		tail, kb.B, err = unmarshalTagValue(kb.B[:0], tail)
		if err != nil {
			return fmt.Errorf("cannot unmarshal tagKey from %X: %s", item, err)
		}

		// Verify that the tag key points to existing metric.
		if len(tail) < 8 {
			return fmt.Errorf("cannot unmarshal metricID from less than 8 bytes; got %d bytes; item=%X", len(tail), tail)
		}
		metricID := encoding.UnmarshalUint64(tail[len(tail)-8:])
		if _, deleted := dmis[metricID]; deleted {
			// The given metric is deleted. Skip it.
			continue
		}

		// Store tag key.
		tks[string(kb.B)] = struct{}{}

		// Search for the next tag key.
		// tkp (tag key prefix) contains (commonPrefix + encoded tag key).
		// The last char must be tagSeparatorChar. Just increment it
		// in order to jump to the next tag key.
		tkp := item[:len(item)-len(tail)]
		if len(tkp) == 0 || tkp[len(tkp)-1] != tagSeparatorChar || tagSeparatorChar >= 0xff {
			logger.Panicf("BUG: the last char in tkp=%X must be %X. Check unmarshalTagValue code", tkp, tagSeparatorChar)
		}
		kb.B = append(kb.B[:0], tkp...)
		kb.B[len(kb.B)-1]++
		ts.Seek(kb.B)
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error during search for commonPrefix %q: %s", commonPrefix, err)
	}
	return nil
}

// SearchTagValues returns all the tag values for the given tagKey
func (db *indexDB) SearchTagValues(accountID, projectID uint32, tagKey []byte, maxTagValues int) ([]string, error) {
	// TODO: cache results?

	kb := kbPool.Get()
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricID, accountID, projectID)
	kb.B = marshalTagValue(kb.B, tagKey)

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

	tagValues := make([]string, 0, len(tvs))
	for tv := range tvs {
		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{}, prefix []byte, maxTagValues int) error {
	ts := &is.ts
	kb := &is.kb
	dmis := is.db.getDeletedMetricIDs()
	ts.Seek(prefix)
	for len(tvs) < maxTagValues && ts.NextItem() {
		k := ts.Item
		if !bytes.HasPrefix(k, prefix) {
			break
		}

		// Get TagValue
		k = k[len(prefix):]
		var err error
		k, kb.B, err = unmarshalTagValue(kb.B[:0], k)
		if err != nil {
			return fmt.Errorf("cannot unmarshal tagValue: %s", err)
		}
		if len(k) != 8 {
			return fmt.Errorf("unexpected suffix after tag value; want %d bytes; got %d bytes", 8, len(k))
		}

		// Verify whether the corresponding metric is deleted.
		if len(dmis) > 0 {
			metricID := encoding.UnmarshalUint64(k)
			if _, deleted := dmis[metricID]; deleted {
				// The metric is deleted.
				continue
			}
		}

		// Store tag value
		tvs[string(kb.B)] = struct{}{}

		// Search for the next tag value.
		// tkp (tag key prefix) contains (commonPrefix + encoded tag value).
		// The last char must be tagSeparatorChar. Just increment it
		// in order to jump to the next tag key.
		tkp := ts.Item[:len(ts.Item)-8]
		if len(tkp) == 0 || tkp[len(tkp)-1] != tagSeparatorChar || tagSeparatorChar >= 0xff {
			logger.Panicf("BUG: the last char in tkp=%X must be %X. Check unmarshalTagValue code", tkp, tagSeparatorChar)
		}
		kb.B = append(kb.B[:0], tkp...)
		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: %s", prefix, err)
	}
	return nil
}

// GetSeriesCount returns the approximate number of unique timeseries for the given (accountID, projectID).
//
// It includes the deleted series too and may count the same series
// up to two times - in db and extDB.
func (db *indexDB) GetSeriesCount(accountID, projectID uint32) (uint64, error) {
	is := db.getIndexSearch()
	n, err := getSeriesCount(accountID, projectID, &is.ts, &is.kb)
	db.putIndexSearch(is)
	if err != nil {
		return 0, err
	}

	var nExt uint64
	ok := db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch()
		nExt, err = getSeriesCount(accountID, projectID, &is.ts, &is.kb)
		extDB.putIndexSearch(is)
	})
	if ok && err != nil {
		return 0, err
	}
	return n + nExt, nil
}

// searchMetricName appends metric name for the given metricID to dst
// and returns the result.
func (db *indexDB) searchMetricName(dst []byte, metricID uint64, accountID, projectID uint32) ([]byte, error) {
	is := db.getIndexSearch()
	dst, err := is.searchMetricName(dst, metricID, accountID, projectID)
	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, accountID, projectID)
		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.
	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
	}
	accountID := tfss[0].accountID
	projectID := tfss[0].projectID

	// Obtain metricIDs to delete.
	is := db.getIndexSearch()
	metricIDs, err := is.searchMetricIDs(tfss, TimeRange{}, 1e9)
	db.putIndexSearch(is)
	if err != nil {
		return 0, err
	}
	if len(metricIDs) == 0 {
		// Nothing to delete
		return 0, nil
	}

	// Mark the found metricIDs as deleted.
	items := getIndexItems()
	for _, metricID := range metricIDs {
		items.B = append(items.B, nsPrefixDeteletedMetricID)
		items.B = encoding.MarshalUint64(items.B, metricID)
		items.Next()
	}
	err = db.tb.AddItems(items.Items)
	putIndexItems(items)
	if err != nil {
		return 0, err
	}
	deletedCount := len(metricIDs)

	// atomically add deleted metricIDs to an inmemory map.
	db.updateDeletedMetricIDs(metricIDs)

	// Reset TagFilters -> TSIDS cache, since it may contain deleted TSIDs.
	_ = db.invalidateTagCache(accountID, projectID)

	// Delete TSIDs in the extDB.
	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: %s", err)
		}
	}
	return deletedCount, nil
}

func (db *indexDB) getDeletedMetricIDs() map[uint64]struct{} {
	return db.deletedMetricIDs.Load().(map[uint64]struct{})
}

func (db *indexDB) setDeletedMetricIDs(dmis map[uint64]struct{}) {
	db.deletedMetricIDs.Store(dmis)
}

func (db *indexDB) updateDeletedMetricIDs(metricIDs []uint64) {
	db.deletedMetricIDsUpdateLock.Lock()
	dmisOld := db.getDeletedMetricIDs()
	dmisNew := make(map[uint64]struct{}, len(dmisOld)+len(metricIDs))
	for metricID := range dmisOld {
		dmisNew[metricID] = struct{}{}
	}
	for _, metricID := range metricIDs {
		dmisNew[metricID] = struct{}{}
	}
	db.setDeletedMetricIDs(dmisNew)
	db.deletedMetricIDsUpdateLock.Unlock()
}

func (is *indexSearch) loadDeletedMetricIDs() (map[uint64]struct{}, error) {
	dmis := make(map[uint64]struct{})
	ts := &is.ts
	kb := &is.kb
	kb.B = append(kb.B[:0], nsPrefixDeteletedMetricID)
	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[metricID] = struct{}{}
	}
	if err := ts.Error(); err != nil {
		return nil, err
	}
	return dmis, nil
}

// searchTSIDs returns 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 = db.marshalTagFiltersKey(tfKeyBuf.B[:0], tfss)
	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) {
		tsids, ok := extDB.getFromTagCache(tfKeyBuf.B)
		if ok {
			extTSIDs = tsids
			return
		}
		is := extDB.getIndexSearch()
		extTSIDs, err = is.searchTSIDs(tfss, tr, maxMetrics)
		extDB.putIndexSearch(is)

		// Do not store found tsids into extDB.tagCache,
		// since they will be stored into outer cache instead.
	}) {
		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: %s", err)
		}
		if len(tail) > 0 {
			return fmt.Errorf("unexpected non-empty tail left after unmarshaling TSID: %X", tail)
		}
		if len(dmis) > 0 {
			// Verify whether the dst is marked as deleted.
			if _, deleted := dmis[dst.MetricID]; deleted {
				// 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: %s", kb.B, err)
	}
	// Nothing found
	return io.EOF
}

func (is *indexSearch) searchMetricName(dst []byte, metricID uint64, accountID, projectID uint32) ([]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, accountID, projectID)
	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: %s", 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) searchTSIDs(tfss []*TagFilters, tr TimeRange, maxMetrics int) ([]TSID, error) {
	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
	accountID := tfss[0].accountID
	projectID := tfss[0].projectID
	for _, metricID := range metricIDs {
		// Try obtaining TSIDs from db.tsidCache. 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(&tsids[i], metricID, accountID, projectID); 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: %s", 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, accountID, projectID uint32) 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, accountID, projectID)
	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: %s", kb.B, err)
	}
	v := ts.Item[len(kb.B):]
	tail, err := dst.Unmarshal(v)
	if err != nil {
		return fmt.Errorf("cannot unmarshal TSID=%X: %s", v, err)
	}
	if len(tail) > 0 {
		return fmt.Errorf("unexpected non-zero tail left after unmarshaling TSID: %X", tail)
	}
	return nil
}

func getSeriesCount(accountID, projectID uint32, ts *mergeset.TableSearch, kb *bytesutil.ByteBuffer) (uint64, error) {
	var n uint64
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixMetricIDToTSID, accountID, projectID)
	ts.Seek(kb.B)
	for ts.NextItem() {
		if !bytes.HasPrefix(ts.Item, kb.B) {
			break
		}
		// Take into account deleted timeseries too.
		n++
	}
	if err := ts.Error(); err != nil {
		return 0, fmt.Errorf("error when counting unique timeseries: %s", err)
	}
	return n, nil
}

// searchMetricIDsMapByMetricNameMatch matches metricName values for the given srcMetricIDs against tfs
// and adds matching metrics to metricIDs.
func (is *indexSearch) searchMetricIDsMapByMetricNameMatch(metricIDs, srcMetricIDs map[uint64]struct{}, tfs []*tagFilter, accountID, projectID uint32) error {
	// sort srcMetricIDs in order to speed up Seek below.
	sortedMetricIDs := make([]uint64, 0, len(srcMetricIDs))
	for metricID := range srcMetricIDs {
		sortedMetricIDs = append(sortedMetricIDs, metricID)
	}
	sort.Slice(sortedMetricIDs, func(i, j int) bool { return sortedMetricIDs[i] < sortedMetricIDs[j] })

	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, accountID, projectID)
		if err != nil {
			return fmt.Errorf("cannot find metricName by metricID %d: %s", metricID, err)
		}
		if err := mn.Unmarshal(metricName.B); err != nil {
			return fmt.Errorf("cannot unmarshal metricName %q: %s", 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: %s", mn, err)
		}
		if !ok {
			continue
		}
		metricIDs[metricID] = struct{}{}
	}
	return nil
}

func (is *indexSearch) getTagFilterWithMinMetricIDsMap(tfs *TagFilters, maxMetrics int) (*tagFilter, map[uint64]struct{}, error) {
	var minMetricIDs map[uint64]struct{}
	var minTf *tagFilter
	for i := range tfs.tfs {
		tf := &tfs.tfs[i]
		if tf.isNegative {
			// Skip negative filters.
			continue
		}
		metricIDs, err := is.getMetricIDsMapForTagFilter(tf, maxMetrics)
		if err != nil {
			if err == errFallbackToMetricNameMatch {
				// Skip tag filters requiring to scan for too many metrics.
				continue
			}
			return nil, nil, fmt.Errorf("cannot find MetricIDs for tagFilter %s: %s", tf, err)
		}
		if minTf == nil || len(metricIDs) < len(minMetricIDs) {
			minMetricIDs = metricIDs
			minTf = tf
			maxMetrics = len(minMetricIDs)
			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
	}

	// There is no positive filter with small number of matching metrics.
	// Create it, so it matches all the MetricIDs for tfs.commonPrefix.
	metricIDs := make(map[uint64]struct{})
	if err := is.getMetricIDsMapForCommonPrefix(metricIDs, tfs.commonPrefix, maxMetrics); err != nil {
		return nil, nil, err
	}
	return nil, metricIDs, nil
}

func matchTagFilters(mn *MetricName, tfs []*tagFilter, kb *bytesutil.ByteBuffer) (bool, error) {
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricID, mn.AccountID, mn.ProjectID)
	for _, 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: %s", mn.MetricGroup, tf, err)
			}
			if !ok {
				return false, nil
			}
			continue
		}

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

			// Found the matching tag name. Match for the value.
			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: %s", tag, tf, err)
			}
			if !ok {
				return false, nil
			}
			tagMatched = true
			break
		}
		if !tagMatched {
			// Matching tag name wasn't found.
			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) {
	metricIDsMap := make(map[uint64]struct{})
	for _, tfs := range tfss {
		if len(tfs.tfs) == 0 {
			// Return all the metric ids
			if err := is.getMetricIDsMapForCommonPrefix(metricIDsMap, tfs.commonPrefix, maxMetrics+1); err != nil {
				return nil, err
			}
			if len(metricIDsMap) > maxMetrics {
				return nil, fmt.Errorf("the number or 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.searchMetricIDsMap(metricIDsMap, tfs, tr, maxMetrics+1); err != nil {
			return nil, err
		}
		if len(metricIDsMap) > maxMetrics {
			return nil, fmt.Errorf("the number or matching unique timeseries exceeds %d; either narrow down the search or increase -search.maxUniqueTimeseries", maxMetrics)
		}
	}
	if len(metricIDsMap) == 0 {
		// Nothing found
		return nil, nil
	}

	metricIDs := getSortedMetricIDs(metricIDsMap)

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

	return metricIDs, nil
}

func (is *indexSearch) searchMetricIDsMap(metricIDs map[uint64]struct{}, tfs *TagFilters, tr TimeRange, maxMetrics int) error {
	// Sort tag filters for faster ts.Seek below.
	sort.Slice(tfs.tfs, func(i, j int) bool { return bytes.Compare(tfs.tfs[i].prefix, tfs.tfs[j].prefix) < 0 })

	// Find the filter with minimum matching metrics.
	// Iteratively increase maxAllowedMetrics up to maxMetrics in order to limit
	// the time required for founding the tag filter with minimum matching metrics.
	var minTf *tagFilter
	var minMetricIDs map[uint64]struct{}
	maxAllowedMetrics := 16
	if maxAllowedMetrics > maxMetrics {
		maxAllowedMetrics = maxMetrics
	}
	for {
		var err error
		minTf, minMetricIDs, err = is.getTagFilterWithMinMetricIDsMap(tfs, maxAllowedMetrics)
		if err != nil {
			return err
		}
		if len(minMetricIDs) < maxAllowedMetrics {
			// Found the tag filter with the minimum number of metrics.
			break
		}

		// Too many metrics matched.
		if maxAllowedMetrics < maxMetrics {
			// Increase maxAllowedMetrics and try again.
			maxAllowedMetrics *= 4
			if maxAllowedMetrics > maxMetrics {
				maxAllowedMetrics = maxMetrics
			}
			continue
		}

		// The tag filter with minimum matching metrics matches at least maxMetrics.
		//
		// 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 below.
		maxTimeRangeMetrics := 20 * maxMetrics
		metricIDsForTimeRange, err := is.getMetricIDsForTimeRange(tr, maxTimeRangeMetrics+1, tfs.accountID, tfs.projectID)
		if err == errMissingMetricIDsForDate {
			// Give up.
			for metricID := range minMetricIDs {
				metricIDs[metricID] = struct{}{}
			}
			return nil
		}
		if err != nil {
			return err
		}
		if len(metricIDsForTimeRange) > maxTimeRangeMetrics {
			return fmt.Errorf("more than %d time series found on the time range %s; either increase -search.maxUniqueTimeseries or shrink the time range",
				maxTimeRangeMetrics, tr.String())
		}
		minMetricIDs = metricIDsForTimeRange
		minTf = nil
		break
	}

	// Find intersection of minTf with other tfs.
	var tfsPostponed []*tagFilter
	for i := range tfs.tfs {
		tf := &tfs.tfs[i]
		if tf == minTf {
			continue
		}
		mIDs, err := is.intersectMetricIDsMapForTagFilter(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
	}
	for i, tf := range tfsPostponed {
		mIDs, err := is.intersectMetricIDsMapForTagFilter(tf, minMetricIDs)
		if err == errFallbackToMetricNameMatch {
			return is.searchMetricIDsMapByMetricNameMatch(metricIDs, minMetricIDs, tfsPostponed[i:], tfs.accountID, tfs.projectID)
		}
		if err != nil {
			return err
		}
		minMetricIDs = mIDs
	}
	for metricID := range minMetricIDs {
		metricIDs[metricID] = struct{}{}
	}
	return nil
}

func (is *indexSearch) getMetricIDsMapForTagFilter(tf *tagFilter, maxMetrics int) (map[uint64]struct{}, error) {
	if tf.isNegative {
		logger.Panicf("BUG: isNegative must be false")
	}
	metricIDs := make(map[uint64]struct{}, maxMetrics)
	if len(tf.orSuffixes) > 0 {
		// Fast path for orSuffixes - seek for rows for each value from orSuffxies.
		if err := is.updateMetricIDsMapForOrSuffixesNoFilter(tf, maxMetrics, metricIDs); err != nil {
			return nil, err
		}
		return metricIDs, nil
	}

	// Slow path - scan all the rows with tf.prefix
	maxLoops := maxMetrics * maxIndexScanLoopsPerMetric
	loops := 0
	ts := &is.ts
	ts.Seek(tf.prefix)
	for len(metricIDs) < maxMetrics && ts.NextItem() {
		loops++
		if loops > maxLoops {
			return nil, errFallbackToMetricNameMatch
		}

		k := ts.Item
		if !bytes.HasPrefix(k, tf.prefix) {
			break
		}

		// Get MetricID from k (the last 8 bytes).
		k = k[len(tf.prefix):]
		if len(k) < 8 {
			return nil, fmt.Errorf("invald key suffix size; want at least %d bytes; got %d bytes", 8, len(k))
		}
		v := k[len(k)-8:]
		k = k[:len(k)-8]

		ok, err := tf.matchSuffix(k)
		if err != nil {
			return nil, fmt.Errorf("error when matching %s: %s", tf, err)
		}
		if !ok {
			continue
		}
		metricID := encoding.UnmarshalUint64(v)
		metricIDs[metricID] = struct{}{}
	}
	if err := ts.Error(); err != nil {
		return nil, fmt.Errorf("error when searching for tag filter prefix %q: %s", tf.prefix, err)
	}
	return metricIDs, nil
}

func (is *indexSearch) updateMetricIDsMapForOrSuffixesNoFilter(tf *tagFilter, maxMetrics int, metricIDs map[uint64]struct{}) 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.updateMetricIDsMapForOrSuffixNoFilter(kb.B, maxMetrics, metricIDs); err != nil {
			return err
		}
		if len(metricIDs) >= maxMetrics {
			return nil
		}
	}
	return nil
}

func (is *indexSearch) updateMetricIDsMapForOrSuffixesWithFilter(tf *tagFilter, metricIDs, filter map[uint64]struct{}) error {
	sortedFilter := getSortedMetricIDs(filter)
	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.updateMetricIDsMapForOrSuffixWithFilter(kb.B, metricIDs, sortedFilter, tf.isNegative); err != nil {
			return err
		}
	}
	return nil
}

func (is *indexSearch) updateMetricIDsMapForOrSuffixNoFilter(prefix []byte, maxMetrics int, metricIDs map[uint64]struct{}) error {
	ts := &is.ts
	maxLoops := maxMetrics * maxIndexScanLoopsPerMetric
	loops := 0
	ts.Seek(prefix)
	for len(metricIDs) < maxMetrics && ts.NextItem() {
		loops++
		if loops > maxLoops {
			return errFallbackToMetricNameMatch
		}
		if !bytes.HasPrefix(ts.Item, prefix) {
			break
		}
		// Get MetricID from ts.Item (the last 8 bytes).
		v := ts.Item[len(prefix):]
		if len(v) != 8 {
			return fmt.Errorf("invalid key suffix size for prefix=%q; want %d bytes; got %d bytes; value=%q", 8, prefix, len(v), v)
		}
		metricID := encoding.UnmarshalUint64(v)
		metricIDs[metricID] = struct{}{}
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for tag filter prefix %q: %s", prefix, err)
	}
	return nil
}

func (is *indexSearch) updateMetricIDsMapForOrSuffixWithFilter(prefix []byte, metricIDs map[uint64]struct{}, sortedFilter []uint64, isNegative bool) error {
	ts := &is.ts
	kb := &is.kb
	for {
		// Seek for the next metricID from sortedFilter.
		if len(sortedFilter) == 0 {
			// All the sorteFilter entries have been searched.
			break
		}
		nextMetricID := sortedFilter[0]
		sortedFilter = sortedFilter[1:]
		kb.B = append(kb.B[:0], prefix...)
		kb.B = encoding.MarshalUint64(kb.B, nextMetricID)
		ts.Seek(kb.B)
		if !ts.NextItem() {
			break
		}
		if !bytes.HasPrefix(ts.Item, prefix) {
			break
		}
		// Get MetricID from ts.Item (the last 8 bytes).
		v := ts.Item[len(prefix):]
		if len(v) != 8 {
			return fmt.Errorf("invalid key suffix size for prefix=%q; want %d bytes; got %d bytes; value=%q", 8, prefix, len(v), v)
		}
		metricID := encoding.UnmarshalUint64(v)
		if metricID != nextMetricID {
			continue
		}
		if isNegative {
			delete(metricIDs, metricID)
			continue
		}
		metricIDs[metricID] = struct{}{}
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for tag filter prefix %q: %s", prefix, err)
	}
	return nil
}

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

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

func (is *indexSearch) getMetricIDsForTimeRange(tr TimeRange, maxMetrics int, accountID, projectID uint32) (map[uint64]struct{}, error) {
	if tr.isZero() {
		return nil, errMissingMetricIDsForDate
	}
	minDate := tr.MinTimestamp / msecPerDay
	maxDate := tr.MaxTimestamp / msecPerDay
	if maxDate-minDate > 40 {
		// Too much dates must be covered. Give up.
		return nil, errMissingMetricIDsForDate
	}
	metricIDs := make(map[uint64]struct{}, maxMetrics)
	for minDate <= maxDate {
		if err := is.getMetricIDsForDate(uint64(minDate), metricIDs, maxMetrics, accountID, projectID); err != nil {
			return nil, err
		}
		minDate++
	}
	return metricIDs, nil
}

func (db *indexDB) storeDateMetricID(date, metricID uint64, accountID, projectID uint32) error {
	is := db.getIndexSearch()
	ok, err := is.hasDateMetricID(date, metricID, accountID, projectID)
	db.putIndexSearch(is)
	if err != nil {
		return err
	}
	if ok {
		// Fast path: the (date, metricID) entry already exists in the db.
		return nil
	}

	// Slow path: create (date, metricID) entry.
	items := getIndexItems()
	items.B = marshalCommonPrefix(items.B[:0], nsPrefixDateToMetricID, accountID, projectID)
	items.B = encoding.MarshalUint64(items.B, date)
	items.B = encoding.MarshalUint64(items.B, metricID)
	items.Next()
	err = db.tb.AddItems(items.Items)
	putIndexItems(items)
	return err
}

func (is *indexSearch) hasDateMetricID(date, metricID uint64, accountID, projectID uint32) (bool, error) {
	ts := &is.ts
	kb := &is.kb
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixDateToMetricID, accountID, projectID)
	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: %s", 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) getMetricIDsForDate(date uint64, metricIDs map[uint64]struct{}, maxMetrics int, accountID, projectID uint32) error {
	ts := &is.ts
	kb := &is.kb
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixDateToMetricID, accountID, projectID)
	kb.B = encoding.MarshalUint64(kb.B, date)
	ts.Seek(kb.B)
	items := 0
	for len(metricIDs) < maxMetrics && ts.NextItem() {
		if !bytes.HasPrefix(ts.Item, kb.B) {
			break
		}
		// Extract MetricID from ts.Item (the last 8 bytes).
		v := ts.Item[len(kb.B):]
		if len(v) != 8 {
			return fmt.Errorf("cannot extract metricID from k; want %d bytes; got %d bytes", 8, len(v))
		}
		metricID := encoding.UnmarshalUint64(v)
		metricIDs[metricID] = struct{}{}
		items++
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for metricIDs for date %d: %s", date, err)
	}
	if items == 0 {
		// There are no metricIDs for the given date.
		// This may be the case for old data when Date -> MetricID wasn't available.
		return errMissingMetricIDsForDate
	}
	return nil
}

func (is *indexSearch) getMetricIDsMapForCommonPrefix(metricIDs map[uint64]struct{}, commonPrefix []byte, maxMetrics int) error {
	ts := &is.ts
	ts.Seek(commonPrefix)
	for len(metricIDs) < maxMetrics && ts.NextItem() {
		k := ts.Item
		if !bytes.HasPrefix(k, commonPrefix) {
			break
		}

		// Extract MetricID from k (the last 8 bytes).
		k = k[len(commonPrefix):]
		if len(k) < 8 {
			return fmt.Errorf("cannot extract metricID from k; want at least %d bytes; got %d bytes", 8, len(k))
		}
		v := k[len(k)-8:]
		metricID := encoding.UnmarshalUint64(v)
		metricIDs[metricID] = struct{}{}
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for metricIDs by commonPrefix %q: %s", commonPrefix, err)
	}
	return nil
}

// The maximum number of index scan loops per already found metric.
// Bigger number of loops is slower than searchMetricIDsMapByMetricNameMatch
// over the found metrics.
const maxIndexScanLoopsPerMetric = 32

func (is *indexSearch) intersectMetricIDsMapForTagFilter(tf *tagFilter, filter map[uint64]struct{}) (map[uint64]struct{}, error) {
	if len(filter) == 0 {
		return nil, nil
	}
	metricIDs := filter
	if !tf.isNegative {
		metricIDs = make(map[uint64]struct{}, len(filter))
	}
	if len(tf.orSuffixes) > 0 {
		// Fast path for orSuffixes - seek for rows for each value from orSuffixes.
		if err := is.updateMetricIDsMapForOrSuffixesWithFilter(tf, metricIDs, filter); err != nil {
			return nil, err
		}
		return metricIDs, nil
	}

	// Slow path - scan for all the rows with the given prefix.
	maxLoops := len(filter) * maxIndexScanLoopsPerMetric
	loops := 0
	ts := &is.ts
	ts.Seek(tf.prefix)
	for ts.NextItem() {
		loops++
		if loops > maxLoops {
			return nil, errFallbackToMetricNameMatch
		}

		k := ts.Item
		if !bytes.HasPrefix(k, tf.prefix) {
			break
		}

		// Extract MetricID from k (the last 8 bytes).
		k = k[len(tf.prefix):]
		if len(k) < 8 {
			return nil, fmt.Errorf("cannot extract metricID from k; want at least %d bytes; got %d bytes", 8, len(k))
		}
		v := k[len(k)-8:]
		k = k[:len(k)-8]

		ok, err := tf.matchSuffix(k)
		if err != nil {
			return nil, fmt.Errorf("error when matching %s: %s", tf, err)
		}
		if !ok {
			continue
		}
		metricID := encoding.UnmarshalUint64(v)
		if tf.isNegative {
			// filter must be equal to metricIDs
			delete(metricIDs, metricID)
			continue
		}
		if _, ok := filter[metricID]; ok {
			metricIDs[metricID] = struct{}{}
		}
	}
	if err := ts.Error(); err != nil {
		return nil, fmt.Errorf("error searching %q: %s", tf.prefix, err)
	}
	return metricIDs, nil
}

var kbPool bytesutil.ByteBufferPool

// Returns local unique MetricID.
func getUniqueUint64() uint64 {
	return atomic.AddUint64(&uniqueUint64, 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 uniqueUint64 = uint64(time.Now().UnixNano())

func marshalCommonPrefix(dst []byte, nsPrefix byte, accountID, projectID uint32) []byte {
	dst = append(dst, nsPrefix)
	dst = encoding.MarshalUint32(dst, accountID)
	dst = encoding.MarshalUint32(dst, projectID)
	return dst
}

func getSortedMetricIDs(m map[uint64]struct{}) []uint64 {
	a := make([]uint64, len(m))
	i := 0
	for metricID := range m {
		a[i] = metricID
		i++
	}
	sort.Slice(a, func(i, j int) bool { return a[i] < a[j] })
	return a
}