diff --git a/docs/sources/query/metric_queries.md b/docs/sources/query/metric_queries.md
index d3d2efb2714d7..153986c2768e4 100644
--- a/docs/sources/query/metric_queries.md
+++ b/docs/sources/query/metric_queries.md
@@ -153,3 +153,14 @@ Examples:
       or
     vector(0) # will return 0
     ```
+
+## Probabilistic aggregation
+
+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, for example, when it uses an aggregation by a structured metadata label.