Github user davies commented on a diff in the pull request:
https://github.com/apache/spark/pull/11828#discussion_r56934685
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -305,21 +305,21 @@ object SetOperationPushDown extends Rule[LogicalPlan]
with PredicateHelper {
}
/**
- * Attempts to eliminate the reading of unneeded columns from the query
plan using the following
- * transformations:
+ * Attempts to eliminate the reading of unneeded columns from the query
plan.
*
- * - Inserting Projections beneath the following operators:
- * - Aggregate
- * - Generate
- * - Project <- Join
- * - LeftSemiJoin
+ * Since adding Project before Filter conflicts with
PushPredicatesThroughProject, this rule will
+ * remove the Project p2 in the following pattern:
+ *
+ * p1 @ Project(_, Filter(_, p2 @ Project(_, child))) if
p2.outputSet.subsetOf(p2.inputSet)
--- End diff --
We need to insert the p2 to pruning the columns further, for example
Project(Filter(Join)), we need p2 to prune the columns from Join.
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