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_r276573822
##########
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)
Review comment:
Can you give a more complete example query for this rewrite? It is
beneficial for readers.
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