Github user gengliangwang commented on a diff in the pull request: https://github.com/apache/spark/pull/18692#discussion_r144466472 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/joins.scala --- @@ -152,3 +152,71 @@ object EliminateOuterJoin extends Rule[LogicalPlan] with PredicateHelper { if (j.joinType == newJoinType) f else Filter(condition, j.copy(joinType = newJoinType)) } } + +/** + * A rule that uses propagated constraints to infer join conditions. The optimization is applicable + * only to CROSS joins. + * + * For instance, if there is a CROSS join, where the left relation has 'a = 1' and the right + * relation has 'b = 1', then the rule infers 'a = b' as a join predicate. + */ +object InferJoinConditionsFromConstraints extends Rule[LogicalPlan] with PredicateHelper { + + def apply(plan: LogicalPlan): LogicalPlan = { + if (SQLConf.get.constraintPropagationEnabled) { + inferJoinConditions(plan) + } else { + plan + } + } + + private def inferJoinConditions(plan: LogicalPlan): LogicalPlan = plan transform { + case join @ Join(left, right, Cross, conditionOpt) => + val leftConstraints = join.constraints.filter(_.references.subsetOf(left.outputSet)) + val rightConstraints = join.constraints.filter(_.references.subsetOf(right.outputSet)) --- End diff -- I don't think we need to separate the constraints as left only and right only. The following case can infer `t1.col1 = t2.col1`: ```scala Seq((1, 2)).toDF("col1", "col2").write.saveAsTable("t1") Seq((1, 2)).toDF("col1", "col2").write.saveAsTable("t2") val df = spark.sql("SELECT * FROM t1 CROSS JOIN t2 ON t1.col1 >= t2.col1 " + "WHERE t1.col1 = t1.col2 + t2.col2 and t2.col1 = t1.col2 + t2.col2") ```
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