[GitHub] [spark] maropu commented on a change in pull request #32559: [SPARK-35410][SQL] SubExpr elimination should not include redundant children exprs in conditional expression

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maropu commented on a change in pull request #32559:
URL: https://github.com/apache/spark/pull/32559#discussion_r633282516



##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/EquivalentExpressions.scala
##########
@@ -82,21 +82,31 @@ class EquivalentExpressions {
   /**
    * Adds only expressions which are common in each of given expressions, in a recursive way.
    * For example, given two expressions `(a + (b + (c + 1)))` and `(d + (e + (c + 1)))`,
-   * the common expression `(c + 1)` will be added into `equivalenceMap`.
+   * the common expression `(c + 1)` will be added into `equivalenceMap`. Note that if an
+   * expression and its child expressions are all commonly occurred in each of given expressions,
+   * we filter out the child expressions. For example, if `((a + b) + c)` and `(a + b)` are
+   * common expressions, we only add `((a + b) + c)`.

Review comment:
       > If the redundant children expressions are counted as common expressions too, they will be redundantly evaluated and miss the subexpression elimination opportunity.
   
   Could you leave comments here about why we need to filter out these exprs here?

##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/EquivalentExpressions.scala
##########
@@ -82,21 +82,31 @@ class EquivalentExpressions {
   /**
    * Adds only expressions which are common in each of given expressions, in a recursive way.
    * For example, given two expressions `(a + (b + (c + 1)))` and `(d + (e + (c + 1)))`,
-   * the common expression `(c + 1)` will be added into `equivalenceMap`.
+   * the common expression `(c + 1)` will be added into `equivalenceMap`. Note that if an
+   * expression and its child expressions are all commonly occurred in each of given expressions,
+   * we filter out the child expressions. For example, if `((a + b) + c)` and `(a + b)` are
+   * common expressions, we only add `((a + b) + c)`.
    */
   private def addCommonExprs(
       exprs: Seq[Expression],
       addFunc: Expression => Boolean = addExpr): Unit = {
     val exprSetForAll = mutable.Set[Expr]()
     addExprTree(exprs.head, addExprToSet(_, exprSetForAll))
 
-    val commonExprSet = exprs.tail.foldLeft(exprSetForAll) { (exprSet, expr) =>
+    val candidateExprs = exprs.tail.foldLeft(exprSetForAll) { (exprSet, expr) =>
       val otherExprSet = mutable.Set[Expr]()
       addExprTree(expr, addExprToSet(_, otherExprSet))
       exprSet.intersect(otherExprSet)
     }
 
-    commonExprSet.foreach(expr => addFunc(expr.e))
+    // Not all expressions in the set should be added. We should filter out the subexprs.
+    val commonExprSet = candidateExprs.filter { candidateExpr =>
+      candidateExprs.forall { expr =>
+        expr == candidateExpr || expr.e.find(_.semanticEquals(candidateExpr.e)).isEmpty
+      }

Review comment:
       Is this loop not expensive? It seems the time-complexity is big-O(`the total number of expr nodes in candidateExprs)` x `(candidateExprs.size)^2 `)?

##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/EquivalentExpressions.scala
##########
@@ -82,21 +82,31 @@ class EquivalentExpressions {
   /**
    * Adds only expressions which are common in each of given expressions, in a recursive way.
    * For example, given two expressions `(a + (b + (c + 1)))` and `(d + (e + (c + 1)))`,
-   * the common expression `(c + 1)` will be added into `equivalenceMap`.
+   * the common expression `(c + 1)` will be added into `equivalenceMap`. Note that if an
+   * expression and its child expressions are all commonly occurred in each of given expressions,
+   * we filter out the child expressions. For example, if `((a + b) + c)` and `(a + b)` are
+   * common expressions, we only add `((a + b) + c)`.

Review comment:
       Just a question; even if we filter out the redundant expr (e.g., `(a + b)` in this case) here, the suboptimal (this PR pointed out) case still can happen if the expr, `(a + b)`, is added as a common one in the other part? I thought a query like this: ` Seq((1, 1, 1)).toDF("a", "b", "c").select(when($"a" + $"b" + $"c" > 0, $"a" + $"b" + $"c").when($"a" + $"b" + $"c" <= 0, $"a" + $"b"))`.




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