Github user dbtsai commented on a diff in the pull request:
https://github.com/apache/spark/pull/21904#discussion_r205963712
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/expressions.scala
---
@@ -416,6 +416,29 @@ object SimplifyConditionals extends Rule[LogicalPlan]
with PredicateHelper {
// these branches can be pruned away
val (h, t) = branches.span(_._1 != TrueLiteral)
CaseWhen( h :+ t.head, None)
+
+ case e @ CaseWhen(branches, _) =>
+ val newBranches = branches.foldLeft(List[(Expression,
Expression)]()) {
+ case (newBranches, branch) =>
+ if (newBranches.exists(_._1.semanticEquals(branch._1))) {
+ // If a condition in a branch is previously seen, this
branch can be pruned.
+ // TODO: In fact, if a condition is a sub-condition of the
previous one,
+ // TODO: it can be pruned. This is less strict and can be
implemented
+ // TODO: by decomposing seen conditions.
+ newBranches
+ } else if (newBranches.nonEmpty &&
newBranches.last._2.semanticEquals(branch._2)) {
+ // If the outputs of two adjacent branches are the same, two
branches can be combined.
+ newBranches.take(newBranches.length - 1)
+ .:+((Or(newBranches.last._1, branch._1),
newBranches.last._2))
--- End diff --
For example, the following case can be benefitted from this rule,
```scala
CaseWhen((UnresolvedAttribute("a"), Literal(1)) ::
(Not(UnresolvedAttribute("a")), Literal(1)) ::
(LessThan(Rand(1), Literal(0.5)), Literal(3)) ::
(NonFoldableLiteral(true), Literal(4)) ::
(NonFoldableLiteral(false), Literal(5)) ::
Nil,
None),
Literal(1))
```
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