[GitHub] [arrow-datafusion] crepererum commented on a diff in pull request #5377: Optimize count_distinct.size

["crepererum (via GitHub)" <gi...@apache.org>]

crepererum commented on code in PR #5377:
URL: https://github.com/apache/arrow-datafusion/pull/5377#discussion_r1116799043


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datafusion/physical-expr/src/aggregate/count_distinct.rs:
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@@ -216,23 +216,19 @@ impl Accumulator for DistinctCountAccumulator {
     }
 
     fn size(&self) -> usize {
+        // temporarily calculating the size approximately, taking first batch size * number of batches
+        // such approach has some inaccuracy for variable length values, like strings.

Review Comment:
   This basically remove proper memory accounting for this operation and for strings will likely be very wrong. I would rather see a proper cached size accounting here.



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