cloud-fan commented on a change in pull request #28733:
URL: https://github.com/apache/spark/pull/28733#discussion_r437882318
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File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala
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@@ -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 " +
Review comment:
shall we return Nil from here?
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