alamb commented on code in PR #5377:
URL: https://github.com/apache/arrow-datafusion/pull/5377#discussion_r1117941665
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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:
> are we missing this conditionall check in this PR? so we still have
accurate size (slow for now) for variable data and accurate size (fast) for
fixed lenth data
This is the middle path I would suggest: keep the slow but accurate
accounting for variable length data (aka strings) and add a fast path for fixed
length sizes (what is in the benchmark)
I believe the the additional overhead of doing accurate size accounting for
string values is a relatively smaller amount of the overall time compared to
fixed size types. Making `count distinct` with a large number of string values
fast is likely going to take a more sophisticated approach to this query in
general.
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