maropu commented on a change in pull request #27058: [SPARK-30276][SQL] Support
Filter expression allows simultaneous use of DISTINCT
URL: https://github.com/apache/spark/pull/27058#discussion_r368807309
##########
File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/RewriteDistinctAggregates.scala
##########
@@ -148,24 +207,106 @@ object RewriteDistinctAggregates extends
Rule[LogicalPlan] {
val distinctAggs = exprs.flatMap { _.collect {
case ae: AggregateExpression if ae.isDistinct => ae
}}
- // We need at least two distinct aggregates for this rule because
aggregation
- // strategy can handle a single distinct group.
+ // This rule serves two purposes:
+ // One is to rewrite when there exists at least two distinct aggregates.
We need at least
+ // two distinct aggregates for this rule because aggregation strategy can
handle a single
+ // distinct group.
+ // Another is to expand distinct aggregates which exists filter clause so
that we can
+ // evaluate the filter locally.
Review comment:
The current statement is duplicate with the comment in L172-178. I think its
ok to simply say;
```
// We need at least two distinct aggregates or a single distinct
aggregate with a filter for
// this rule because aggregation strategy can handle a single distinct
group without a filter.
// This check can produce false-positives, e.g., SUM(DISTINCT a) &
COUNT(DISTINCT a).
```
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