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

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viirya commented on a change in pull request #28733:
URL: https://github.com/apache/spark/pull/28733#discussion_r436234167



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File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/PushCNFPredicateThroughJoin.scala
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@@ -0,0 +1,145 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.catalyst.optimizer
+
+import scala.collection.mutable
+
+import org.apache.spark.sql.catalyst.expressions.{And, Expression, Not, Or, PredicateHelper}
+import org.apache.spark.sql.catalyst.plans._
+import org.apache.spark.sql.catalyst.plans.logical.{Filter, Join, LogicalPlan}
+import org.apache.spark.sql.catalyst.rules.Rule
+import org.apache.spark.sql.internal.SQLConf
+
+/**
+ * Try converting join condition to conjunctive normal form expression so that more predicates may
+ * be able to be pushed down.
+ * To avoid expanding the join condition, the join condition will be kept in the original form even
+ * when predicate pushdown happens.
+ */
+object PushCNFPredicateThroughJoin extends Rule[LogicalPlan] with PredicateHelper {
+  /**
+   * 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.
+   */
+  protected def conjunctiveNormalForm(condition: Expression): Seq[Expression] = {

Review comment:
       Could you add tests for this optimization rule? We should have particular tests to verify the CNF conversion.




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