dongjoon-hyun commented on code in PR #55628:
URL: https://github.com/apache/spark/pull/55628#discussion_r3170397734
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sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/PlanMerger.scala:
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@@ -91,6 +92,11 @@ object PlanMerger {
* When plans also differ in intermediate [[Project]] expressions, those are
wrapped with
* `If(filterAttr, expr, null)` to avoid computing the expression for rows
that do not
* match that side's filter condition.
+ * Filter propagation also works through [[Join]] nodes: a filter on one child
of the join
+ * produces a boolean attribute that flows through the join output to the
enclosing
+ * [[Aggregate]]. Propagation is skipped when both the left and right children
simultaneously
+ * produce filter attributes, as combining them would require an additional
AND alias above
+ * the join (not yet supported).
Review Comment:
According to the `filterSafeForJoin`, shall we mention NULL-pad cases a
little more?
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