[GitHub] [spark] wangyum commented on a change in pull request #28805: [SPARK-28169][SQL] Convert scan predicate condition to CNF

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

wangyum commented on a change in pull request #28805:
URL: https://github.com/apache/spark/pull/28805#discussion_r439236301



##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/predicates.scala
##########
@@ -253,6 +253,58 @@ trait PredicateHelper extends Logging {
     expressions.groupBy(_.references.map(_.qualifier)).map(_._2.reduceLeft(And)).toSeq
   }
 
+  /**
+   * Convert an expression into conjunctive normal form for partition pruning.
+   * 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 [[SQLConf.MAX_CNF_NODE_COUNT]] to prevent such cases.
+   *
+   * @param condition to be converted into CNF.
+   * @return the CNF result as sequence of disjunctive expressions. If the number of expressions
+   *         exceeds threshold on converting `Or`, `Seq.empty` is returned.
+   */
+  def conjunctiveNormalFormForPartitionPruning(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 = resultStack.pop()
+          val left = 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 qualifier as one single predicate,
+          // for reducing the size of pushed down predicates and corresponding codegen.
+          val right = groupExpressionsByReference(resultStack.pop())
+          val left = groupExpressionsByReference(resultStack.pop())
+          // Stop the loop whenever the result exceeds the `maxCnfNodeCount`
+          if (left.size * right.size > maxCnfNodeCount) {
+            logInfo(s"As the result size exceeds the threshold $maxCnfNodeCount. " +
+              "The CNF conversion is skipped and returning Seq.empty now. To avoid this, you can " +
+              s"raise the limit ${SQLConf.MAX_CNF_NODE_COUNT.key}.")
+            return Seq.empty
+          } else {
+            for { x <- left; y <- right } yield Or(x, y)
+          }
+        case other => other :: Nil
+      }
+      resultStack.push(cnf)
+    }
+    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.")
+      return Seq.empty
+    }
+    resultStack.top
+  }
+
+  private def groupExpressionsByReference(expressions: Seq[Expression]): Seq[Expression] = {
+    expressions.groupBy(_.references).map(_._2.reduceLeft(And)).toSeq
+  }
+

Review comment:
       Too many duplicate code here. Maybe you can organize code:
   ```scala
     def conjunctiveNormalFormAndGroupExpsByQualifier(condition: Expression): Seq[Expression] = {
       conjunctiveNormalForm(condition,
         (expressions: Seq[Expression]) =>
           expressions.groupBy(_.references.map(_.qualifier)).map(_._2.reduceLeft(And)).toSeq)
     }
   
     def conjunctiveNormalFormAndGroupExpsByReference(condition: Expression): Seq[Expression] = {
       conjunctiveNormalForm(condition,
         (expressions: Seq[Expression]) =>
           expressions.groupBy(_.references).map(_._2.reduceLeft(And)).toSeq)
     }
   
     def conjunctiveNormalForm(
         condition: Expression,
         groupExpsFunc: Seq[Expression] => Seq[Expression] = _.toSeq): 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 = resultStack.pop()
             val left = 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 qualifier as one single predicate,
             // for reducing the size of pushed down predicates and corresponding codegen.
             val right = groupExpsFunc(resultStack.pop())
             val left = groupExpsFunc(resultStack.pop())
             // Stop the loop whenever the result exceeds the `maxCnfNodeCount`
             if (left.size * right.size > maxCnfNodeCount) {
               logInfo(s"As the result size exceeds the threshold $maxCnfNodeCount. " +
                 "The CNF conversion is skipped and returning Seq.empty now. To avoid this, you can " +
                 s"raise the limit ${SQLConf.MAX_CNF_NODE_COUNT.key}.")
               return Seq.empty
             } else {
               for { x <- left; y <- right } yield Or(x, y)
             }
           case other => other :: Nil
         }
         resultStack.push(cnf)
       }
       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.")
         return Seq.empty
       }
       resultStack.top
     }
   ```




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