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docs/MetricsQL.md: document that input histograms must have the same set of buckets when calculating the quantile over them

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Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3231
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Aliaksandr Valialkin 2022-10-14 13:59:56 +03:00
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@ -1022,14 +1022,18 @@ per each `job` over the last 5 minutes.
#### histogram_quantile
`histogram_quantile(phi, buckets)` is a [transform function](#transform-functions), which calculates `phi`-quantile over the given
[histogram buckets](https://valyala.medium.com/improving-histogram-usability-for-prometheus-and-grafana-bc7e5df0e350).
`histogram_quantile(phi, buckets)` is a [transform function](#transform-functions), which calculates `phi`-[percentile](https://en.wikipedia.org/wiki/Percentile)
over the given [histogram buckets](https://valyala.medium.com/improving-histogram-usability-for-prometheus-and-grafana-bc7e5df0e350).
`phi` must be in the range `[0...1]`. For example, `histogram_quantile(0.5, sum(rate(http_request_duration_seconds_bucket[5m]) by (le))`
would return median request duration for all the requests during the last 5 minutes.
The function accepts optional third arg - `boundsLabel`. In this case it returns `lower` and `upper` bounds for the estimated percentile with the given `boundsLabel` label.
See [this issue for details](https://github.com/prometheus/prometheus/issues/5706).
When the [percentile](https://en.wikipedia.org/wiki/Percentile) is calculated over multiple histograms,
then all the input histograms **must** have buckets with identical boundaries, e.g. they must have the same set of `le` or `vmrange` labels.
Otherwise the returned result may be invalid. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3231) for details.
This function is supported by PromQL (except of the `boundLabel` arg). See also [histogram_quantiles](#histogram_quantiles), [histogram_share](#histogram_share)
and [quantile](#quantile).