viirya commented on a change in pull request #24344: [SPARK-27440][SQL]
Optimize uncorrelated predicate subquery
URL: https://github.com/apache/spark/pull/24344#discussion_r276574876
##########
File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/subquery.scala
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@@ -551,3 +552,30 @@ object RewriteCorrelatedScalarSubquery extends
Rule[LogicalPlan] {
}
}
}
+
+/**
+ * This rule rewrites uncorrelated PredicateSubquery expressions such as
Exists.
+ * The uncorrelated Exists can be evaluated using a subplan instead of a
semi-join.
+ * Also, we can use `limit 1` and `select 1` after the subquery to reduce the
result set.
+ * Example:
+ * exists(select b from t where t.a = 2) => exists(select 1 from t where t.a =
2 limit 1)
+ */
+object RewriteUncorrelatedSubquery extends Rule[LogicalPlan] {
+ /**
+ * Wrap the subquery with `limit 1` and `project 1`.
+ */
+ def buildSubquery(sub: LogicalPlan): LogicalPlan = {
+ Project(Seq(Alias(Literal.create(1, IntegerType), "1")()),
+ Limit(Literal.create(1, IntegerType), sub))
+ }
+
+ override def apply(plan: LogicalPlan): LogicalPlan = plan transform {
+ case Filter(condition, child) =>
+ val newCondition = condition transform {
+ case Exists(sub, children, _) if children.isEmpty =>
Review comment:
Only deal with `Exists`? Will it process `InSubquery`?
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