Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/16659#discussion_r97202843
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/EquivalentExpressions.scala
---
@@ -67,28 +67,33 @@ class EquivalentExpressions {
/**
* Adds the expression to this data structure recursively. Stops if a
matching expression
* is found. That is, if `expr` has already been added, its children are
not added.
- * If ignoreLeaf is true, leaf nodes are ignored.
*/
- def addExprTree(
- root: Expression,
- ignoreLeaf: Boolean = true,
- skipReferenceToExpressions: Boolean = true): Unit = {
- val skip = (root.isInstanceOf[LeafExpression] && ignoreLeaf) ||
+ def addExprTree(expr: Expression): Unit = {
+ val skip = expr.isInstanceOf[LeafExpression] ||
// `LambdaVariable` is usually used as a loop variable, which can't
be evaluated ahead of the
// loop. So we can't evaluate sub-expressions containing
`LambdaVariable` at the beginning.
- root.find(_.isInstanceOf[LambdaVariable]).isDefined
- // There are some special expressions that we should not recurse into
children.
+ expr.find(_.isInstanceOf[LambdaVariable]).isDefined
+
+ // There are some special expressions that we should not recurse into
all of its children.
// 1. CodegenFallback: it's children will not be used to generate
code (call eval() instead)
- // 2. ReferenceToExpressions: it's kind of an explicit
sub-expression elimination.
- val shouldRecurse = root match {
- // TODO: some expressions implements `CodegenFallback` but can still
do codegen,
- // e.g. `CaseWhen`, we should support them.
- case _: CodegenFallback => false
- case _: ReferenceToExpressions if skipReferenceToExpressions => false
- case _ => true
+ // 2. If: common subexpressions will always be evaluated at the
beginning, but the true and
+ // false expressions in `If` may not get accessed, according
to the predicate
+ // expression. We should only recurse into the predicate
expression.
+ // 3. CaseWhen: like `If`, the children of `CaseWhen` only get
accessed in a certain
+ // condition. We should only recurse into the first
condition expression as it
+ // will always get accessed.
--- End diff --
`CaseWhen` could be very deep.
> CASE WHEN expr1 THEN expr2 [WHEN expr3 THEN expr4]* [ELSE expr5] END
> When `expr1` = true, returns `expr2`; when `expr3` = true, return
`expr4`; else return `expr5`.
Compared with the previous impl, will we miss some expression elimination
chances?
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