Dandandan commented on code in PR #5377:
URL: https://github.com/apache/arrow-datafusion/pull/5377#discussion_r1117945872
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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:
The overhead for an accurate answer is still enormous for variable length
types and gets worse with increasing table sizes, as the `size` gets slower
with more distinct values in the aggregation, and is called for every update
(so a `O(n^2)` operation).
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