neilconway commented on code in PR #22530:
URL: https://github.com/apache/datafusion/pull/22530#discussion_r3305935348
##########
datafusion/common/src/config.rs:
##########
@@ -1124,6 +1124,13 @@ config_namespace! {
/// into the file scan phase.
pub enable_topk_dynamic_filter_pushdown: bool, default = true
+ /// When set to true, uncorrelated scalar subqueries are left in
+ /// the logical plan and executed by `ScalarSubqueryExec` during
physical
+ /// execution. When set to false, all scalar subqueries (including
+ /// uncorrelated ones) are rewritten to left joins by the
+ /// `ScalarSubqueryToJoin` optimizer rule.
+ pub physical_uncorrelated_scalar_subquery: bool, default = true
Review Comment:
The other similar config options use the phrasing `enable_...`; we should
probably adopt that for consistency.
`physical_uncorrelated_scalar_subquery` is also a mouthful, although I can't
immediately think of a more concise name that is also accurate.
##########
datafusion/sqllogictest/test_files/subquery.slt:
##########
@@ -2091,6 +2091,95 @@ SELECT (SELECT v FROM (SELECT 1 AS v UNION ALL SELECT 2)
AS t ORDER BY v LIMIT 1
----
1
+#############
+## End-to-end correctness coverage for the flag-off path.
+## When `datafusion.optimizer.physical_uncorrelated_scalar_subquery` is false,
+## uncorrelated scalar subqueries are rewritten to left joins by
+## `ScalarSubqueryToJoin` instead of executed by `ScalarSubqueryExec`. This
+## restores pre-PR-21240 behavior, which has three known shortcomings the
Review Comment:
I think you only list two known shortcomings?
##########
datafusion/optimizer/src/scalar_subquery_to_join.rs:
##########
Review Comment:
This comment needs updating.
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