Fix full merge join gang size crash in ORCA - #1896
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CPhysicalFullMergeJoin::PdsDerive unconditionally returned the outer child's distribution spec whenever it was Universal, ignoring the inner child's real distribution. When the inner child was actually Singleton (gathered to combine with a Universal/empty outer side), the join's output was mislabeled Universal. Universal trivially satisfies a Replicated requirement, so an enclosing per-segment join (e.g. a comma-join with a third relation) skipped the corrective motion this Singleton child needed, nesting a real cross-segment Gather Motion inside a per-segment execution context. The executor's sanity check in ExecInitMotion (nodeMotion.c) caught the inconsistent slice and raised "unexpected gang size: N". Fix by mirroring the already-correct sibling implementations (CPhysicalFullHashJoin::PdsDerive, CPhysicalHashJoin:: PdsDeriveForOuterJoin): propagate the inner child's real distribution instead of blindly returning the outer's.
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CPhysicalFullMergeJoin::PdsDerive unconditionally returned the outer child's distribution spec whenever it was Universal, ignoring the inner child's real distribution. When the inner child was actually Singleton (gathered to combine with a Universal/empty outer side), the join's output was mislabeled Universal.
Universal trivially satisfies a Replicated requirement, so an enclosing per-segment join (e.g. a comma-join with a third relation) skipped the corrective motion this Singleton child needed, nesting a real cross-segment Gather Motion inside a per-segment execution context. The executor's sanity check in ExecInitMotion (nodeMotion.c) caught the inconsistent slice and raised "unexpected gang size: N".
Fix by mirroring the already-correct sibling implementations (CPhysicalFullHashJoin::PdsDerive, CPhysicalHashJoin:: PdsDeriveForOuterJoin): propagate the inner child's real distribution instead of blindly returning the outer's.
Fixes #ISSUE_Number
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make installcheckmake -C src/test installcheck-cbdb-parallelImpact
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