[branch-54] Gate new ScalarSubqueryExec node behind session property (#22530)#22690
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[branch-54] Gate new ScalarSubqueryExec node behind session property (#22530)#22690LiaCastaneda wants to merge 2 commits into
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Related to discussion on apache#21240 and apache#21080 (comment). PR apache#21240 introduced `ScalarSubqueryExec` / `ScalarSubqueryExpr` to execute uncorrelated scalar subqueries during physical execution. The two communicate via shared in process state (a `slot` in `ExecutionProps`), which breaks distributed execution that may split execution across a network boundary between the producer (`ScalarSubqueryExec`) and the consumer expression (`ScalarSubqueryExpr`). See more details on this explanation in [datafusion-contrib/datafusion-distributed#460](datafusion-contrib/datafusion-distributed#460) Adds a new optimizer config option `datafusion.optimizer.enable_physical_uncorrelated_scalar_subquery` (default true, preserving the current behavior). When true (default), behavior is unchanged from current main; when false, all scalar subqueries are rewritten to left joins by `ScalarSubqueryToJoin` and `ScalarSubqueryExec` is never constructed (which was the previous behavior). Yes all tests pass and added `uncorrelated_scalar_subquery_rewritten_when_flag_off` to test the negative case. Yes, a new config option `datafusion.optimizer.physical_uncorrelated_scalar_subquery` (this just changes the way the query is executed but not the results)
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| # Scalar subquery returning zero rows → NULL | ||
| query I | ||
| SELECT (SELECT v FROM sq_empty); | ||
| ---- | ||
| NULL |
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on branch-54 this query returns nothing (empty), on main it returns NULL because of https://github.com/apache/datafusion/pull/22316/changes. I think this is a preexisting bug (that the PR i mention fixed), the correct behavior is to return null
Do we want to backport that PR as well?
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Yeah, I think we should backport #22316 as well
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(this is a test case I added on the PR I'm backporting, so this bug will not pop up in the existing sqllogic tests)
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I just confirmed this bug is preexisting, this was the behavior in branch-53
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Which issue does this PR close?
Rationale for this change
This PR backports #22530
What changes are included in this PR?
Are these changes tested?
Are there any user-facing changes?