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Commitd1ce745

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Doc: add comment about bug fixed in back branches as of3f7323c.
While the bug I just fixed in the back branches doesn't exist inHEAD, the requirement that MULTIEXPR SubPlans not share outputparameters still does. Add a comment to memorialize that, becauseperhaps it could be an issue again someday.Discussion:https://postgr.es/m/17596-c5357f61427a81dc@postgresql.org
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‎src/backend/executor/nodeSubplan.c

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@@ -245,6 +245,21 @@ ExecScanSubPlan(SubPlanState *node,
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* ones, so this should be safe.) Unlike ExecReScanSetParamPlan, we do
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* *not* set bits in the parent plan node's chgParam, because we don't
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* want to cause a rescan of the parent.
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*
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* Note: we are also relying on MULTIEXPR SubPlans not sharing any output
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* parameters with other SubPlans, because if one does then it is unclear
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* which SubPlanState node the parameter's execPlan field will be pointing
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* to when we come to evaluate the parameter. We can allow plain initplan
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* SubPlans to share output parameters, because it doesn't actually matter
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* which initplan SubPlan we reference as long as they all point to the
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* same underlying subplan. However, that fails to hold for MULTIEXPRs
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* because they can have non-empty args lists, and the "same" args might
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* have mutated into different forms in different parts of a plan tree.
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* There is currently no problem because MULTIEXPR can appear only in an
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* UPDATE's top-level target list, so it won't get duplicated anyplace.
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* Postgres versions before v14 had to make concrete efforts to avoid
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* sharing output parameters across different clones of a MULTIEXPR, and
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* the problem could recur someday.
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*/
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if (subLinkType==MULTIEXPR_SUBLINK)
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{

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