[GitHub] [druid] gianm commented on a change in pull request #10285: Add note about aggregations on floats

[GitBox <gi...@apache.org>]

gianm commented on a change in pull request #10285:
URL: https://github.com/apache/druid/pull/10285#discussion_r470809516



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File path: docs/development/extensions-core/stats.md
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@@ -44,6 +44,13 @@ This algorithm was proven to be numerically stable by J.L. Barlow in
 "Error analysis of a pairwise summation algorithm to compute sample variance"
 Numer. Math, 58 (1991) pp. 583--590
 
+> As with all [aggregators](../../querying/sql.md#aggregation-functions), the order of operations across segments is
+> non-deterministic. This means that if this aggregator is used with an input type of "float", the result of the
+> aggregation will not be precisely the same across multiple runs of the query.
+>
+> To produce consistent results, either cast the input type to a "double" or round the variance to

Review comment:
       Double and float are both floating-point, so should behave the same way in this regard.

##########
File path: docs/querying/sql.md
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@@ -232,6 +232,10 @@ possible for two aggregators in the same SQL query to have different filters.
 
 Only the COUNT aggregation can accept DISTINCT.
 
+> The order of aggregation operations across segments is not deterministic. This means that non-commutative aggregation
+> functions can produce inconsistent results across the same query. Any functions that operate on an input type of "float"
+> may also see these differences in aggregation results across multiple query runs.

Review comment:
       Similar comment.




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