[GitHub] spark pull request: [SPARK-6624][WIP] Draft of another alternative...

[liancheng <gi...@git.apache.org>]

Github user liancheng commented on a diff in the pull request:

    https://github.com/apache/spark/pull/10444#discussion_r48332590
  
    --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala ---
    @@ -47,6 +48,34 @@ trait Predicate extends Expression {
       override def dataType: DataType = BooleanType
     }
     
    +object Predicate extends PredicateHelper {
    +  def toCNF(predicate: Expression, maybeThreshold: Option[Double] = None): Expression = {
    +    val cnf = new CNFExecutor(predicate).execute(predicate)
    +    val threshold = maybeThreshold.map(predicate.size * _).getOrElse(Double.MaxValue)
    +    if (cnf.size > threshold) predicate else cnf
    --- End diff --
    
    When the threshold is exceeded, the original predicate rather than the intermediate converted predicate is returned. This because the intermediate result may not be in CNF, thus:
    
    1. It doesn't bring much benefit for filter push-down, and
    2. It's much larger than the original predicate and brings extra evaluation cost.


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