francis0407 commented on a change in pull request #24344: [SPARK-27440][SQL]
Optimize uncorrelated predicate subquery
URL: https://github.com/apache/spark/pull/24344#discussion_r278608147
##########
File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/subquery.scala
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@@ -551,3 +552,47 @@ object RewriteCorrelatedScalarSubquery extends
Rule[LogicalPlan] {
}
}
}
+
+/**
+ * This rule rewrites uncorrelated PredicateSubquery expressions such as
Exists, InSubquery.
+ * The uncorrelated Exists and InSubquery can be evaluated using a subplan
instead of a semi-join.
+ *
+ * For uncorrelated Exists, we can use `limit 1` and `select 1` after the
Exists subquery to
+ * reduce the result set.
+ * {{{
+ * SELECT * FROM s WHERE EXISTS(SELECT b FROM t WHERE t.a = 2);
+ * ==> SELECT * FROM s WHERE EXISTS(SELECT 1 FROM t WHERE t.a = 2 LIMIT 1);
+ * }}}
+ *
+ * For uncorrelated InSubquery, we can push the left values into the subquery
to reduce the result
+ * set. Note that InSubquery may be nullable, so we can not eliminate nulls
for both sides.
+ * {{{
+ * SELECT * FROM s WHERE 3 IN (SELECT b FROM t WHERE a = 2);
+ * ==> SELECT * FROM s WHERE 3 IN (SELECT b FROM t WHERE a = 2 AND (b = 3 or
b IS NULL));
Review comment:
Yes, I think so.
My main purpose is to ensure the `IN` expression can be evaluated before the
final execution.
If the left values have an attribute, like:
```sql
select *
from t
where t.a in (select b from s)
```
Firstly, we cannot push `t.a` into the subquery. But we still can collect
the subquery before the final execution. If we do that, we may rewrite the
expression as a `InSet`, and broadcast the subquery's result set to each node.
It is just equivalent to using a semi-join. Therefore, I think there is no
need to rewrite a InSubquery with non-literal left values.
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