viirya commented on a change in pull request #25717: [SPARK-29013][SQL]
Structurally equivalent subexpression elimination
URL: https://github.com/apache/spark/pull/25717#discussion_r324869797
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File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/EquivalentExpressions.scala
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@@ -65,35 +92,96 @@ 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.
+ * Adds each expression to structural expression data structure, grouping
them with existing
+ * structurally equivalent expressions. Non-recursive. Returns false if this
doesn't add input
+ * expression actually.
+ */
+ def addStructExpr(expr: Expression): Boolean = {
+ if (expr.deterministic) {
+ // For structural equivalent expressions, we need to pass in int type
ordinals into
+ // split functions. If the number of ordinals is more than JVM function
limit, we skip
+ // this expression.
+ // We calculate function parameter length by the number of ints plus
`INPUT_ROW` plus
+ // a int type result array index.
+ val refs = expr.collect {
+ case _: BoundReference => Literal(0)
+ }
+ val parameterLength = CodeGenerator.calculateParamLength(refs) + 2
+ if (CodeGenerator.isValidParamLength(parameterLength)) {
+ val e: StructuralExpr = StructuralExpr(expr)
+ val f = structEquivalenceMap.get(e)
+ if (f.isDefined) {
+ addExpr(expr, f.get)
+ } else {
+ val exprMap = mutable.HashMap.empty[Expr,
mutable.ArrayBuffer[Expression]]
+ addExpr(expr, exprMap)
+ structEquivalenceMap.put(e, exprMap)
+ }
+ true
+ } else {
+ false
+ }
+ } else {
+ false
+ }
+ }
+
+ /**
+ * Checks if we skip add sub-expressions for given expression.
*/
- def addExprTree(expr: Expression): Unit = {
- val skip = expr.isInstanceOf[LeafExpression] ||
+ private def skipExpr(expr: Expression): Boolean = {
+ 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.
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. 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.
+ // 4. Coalesce: it's also a conditional expression, we should only recurse
into the first
+ // children, because others may not get accessed.
Review comment:
These words are copied from previous place. Let me add one for it.
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