fix: Nested self-referential CASE chains should not cause exponential hashing work during physical planning.#22175
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Nested self-referential CASE chains cause exponential hash work during physical planning because CaseExpr derives Hash over both the original CaseBody and the derived ProjectedCaseBody in EvalMethod. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
EvalMethod (including ProjectedCaseBody) is deterministically derived from CaseBody in try_new(), so hashing it is redundant. With the derived Hash, nested CASE chains caused exponential hash work during ProjectionExec::compute_properties because each level doubled the tree traversal. Manual Hash/Eq on body-only fixes this. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
rluvaton
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@alamb this doesn't look related to our change... do you know who to talk to about CI issues? |
Dandandan
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| // eval_method is functionally derived from body, so excluding it from | ||
| // Hash/Eq avoids redundantly hashing the expression tree twice. For | ||
| // nested CASE chains this prevents exponential blowup (see #22173). |
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| // nested CASE chains this prevents exponential blowup (see #22173). | |
| // nested CASE chains this prevents exponential blowup (see https://github.com/apache/datafusion/issues/22173). |
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Restarted - seems some flaky thing |
Dandandan
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FYI @pepijnve I wonder if there is any way to test for this (a benchmark in sql_planner perhaps) so we don't break it in the future |
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Dang completely missed that. Would it make sense to use something like |
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I think in general we are trying to keep our (already quite extensive) dependency chain low |
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Which issue does this PR close?
Rationale for this change
Explained in issue
What changes are included in this PR?
eval_methodfieldAre these changes tested?
A unit test was added
Are there any user-facing changes?
Pathologically nested CASE/WHEN queries will plan significantly faster.