[GitHub] [spark] cloud-fan commented on a change in pull request #28733: [SPARK-31705][SQL] Push more possible predicates through Join via CNF conversion

[GitBox <gi...@apache.org>]

cloud-fan commented on a change in pull request #28733:
URL: https://github.com/apache/spark/pull/28733#discussion_r437879694



##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala
##########
@@ -198,6 +200,90 @@ trait PredicateHelper {
     case e: Unevaluable => false
     case e => e.children.forall(canEvaluateWithinJoin)
   }
+
+  /**
+   * Convert an expression into conjunctive normal form.
+   * Definition and algorithm: https://en.wikipedia.org/wiki/Conjunctive_normal_form
+   * CNF can explode exponentially in the size of the input expression when converting Or clauses.
+   * Use a configuration MAX_CNF_NODE_COUNT to prevent such cases.
+   *
+   * @param condition to be conversed into CNF.
+   * @return If the number of expressions exceeds threshold on converting Or, return Seq.empty.
+   *         If the conversion repeatedly expands nondeterministic expressions, return Seq.empty.
+   *         Otherwise, return the converted result as sequence of disjunctive expressions.
+   */
+  def conjunctiveNormalForm(condition: Expression): Seq[Expression] = {
+    val postOrderNodes = postOrderTraversal(condition)
+    val resultStack = new mutable.Stack[Seq[Expression]]
+    val maxCnfNodeCount = SQLConf.get.maxCnfNodeCount
+    // Bottom up approach to get CNF of sub-expressions
+    while (postOrderNodes.nonEmpty) {
+      val cnf = postOrderNodes.pop() match {
+        case _: And =>
+          val right: Seq[Expression] = resultStack.pop()
+          val left: Seq[Expression] = resultStack.pop()
+          left ++ right
+        case _: Or =>
+          // For each side, there is no need to expand predicates of the same references.
+          // So here we can aggregate predicates of the same references as one single predicate,
+          // for reducing the size of pushed down predicates and corresponding codegen.
+          val right = aggregateExpressionsOfSameQualifiers(resultStack.pop())
+          val left = aggregateExpressionsOfSameQualifiers(resultStack.pop())
+          // Stop the loop whenever the result exceeds the `maxCnfNodeCount`
+          if (left.size * right.size > maxCnfNodeCount) {
+            Seq.empty
+          } else {
+            for {x <- left; y <- right} yield Or(x, y)
+          }
+        case other => other :: Nil
+      }
+      if (cnf.isEmpty) {
+        return Seq.empty
+      }
+      if (resultStack.length != 1) {
+        logWarning("The length of CNF conversion result stack is supposed to be 1. There might " +
+          "be something wrong with CNF conversion.")
+      }
+      resultStack.push(cnf)
+    }
+    resultStack.top
+  }
+
+  private def aggregateExpressionsOfSameQualifiers(

Review comment:
       nit: `groupByExprsByQualifier`




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