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chore: Document approx_topk keyword. #15179

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chore: Document approx_topk keyword. #15179

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11 changes: 11 additions & 0 deletions docs/sources/query/metric_queries.md
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Expand Up @@ -153,3 +153,14 @@ Examples:
or
vector(0) # will return 0
```

## Probabilistic aggregation

LogQL also supports a probabilistic `topk` approximation that is a drop-in replacement when `topk` hits the maximum series limit.
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The current text assumes that users are already familiar with what topk is, but we don't really do a good job of defining it in the documentation.

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LogQL also supports a probabilistic `topk` approximation that is a drop-in replacement when `topk` hits the maximum series limit.
The `topk` keyword lets you find the largest 1,000 elements in a data stream by sample size. When `topk` hits the maximum series limit, LogQL also supports using a probable approximation; `approx_topk` is a drop-in replacement when `topk` hits the maximum series limit.


```logql
approx_topk(k, <vector expression>)
```

It is only supported for instant queries and does not support grouping. It is useful when the cardinality of the inner
vector is too high, e.g. when it uses an aggregation by a structured metadata label.
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vector is too high, e.g. when it uses an aggregation by a structured metadata label.
vector is too high, for example, when it uses an aggregation by a structured metadata label.