Github user liancheng commented on a diff in the pull request:
https://github.com/apache/spark/pull/8200#discussion_r48328448
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -464,6 +465,24 @@ object OptimizeIn extends Rule[LogicalPlan] {
}
/**
+ * Convert an expression into its conjunctive normal form (CNF), i.e. AND
of ORs.
+ * For example, a && b || c is normalized to (a || c) && (b || c) by this
method.
+ *
+ * Refer to https://en.wikipedia.org/wiki/Conjunctive_normal_form for
more information
+ */
+object CNFNormalization extends Rule[LogicalPlan] {
--- End diff --
How about using the following heuristic solution to prevent exponential
explosion:
1. Add a simple `size` method to `TreeNode`, which returns the size (total
number of nodes) of a tree:
```scala
def size: Int = 1 + children.map(_.size).sum
```
1. Gives up CNF conversion once the result predicate exceeds a predefined
threshold.
For example, we can stop if the size of the converted predicate is 10
times larger than the original one.
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