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Fix subquery pullup to wrap a PlaceHolderVar around the entire RowExpr
that's generated for a whole-row Var referencing the subquery, when the subquery is in the nullable side of an outer join. The previous coding instead put PlaceHolderVars around the elements of the RowExpr. The effect was that when the outer join made the subquery outputs go to null, the whole-row Var produced ROW(NULL,NULL,...) rather than just NULL. There are arguments afoot about whether those things ought to be semantically indistinguishable, but for the moment they are not entirely so, and the planner needs to take care that its machinations preserve the difference. Per bug #5025. Making this feasible required refactoring ResolveNew() to allow more caller control over what is substituted for a Var. I chose to make ResolveNew() a wrapper around a new general-purpose function replace_rte_variables(). I also fixed the ancient bogosity that ResolveNew might fail to set a query's hasSubLinks field after inserting a SubLink in it. Although all current callers make sure that happens anyway, we've had bugs of that sort before, and it seemed like a good time to install a proper solution. Back-patch to 8.4. The problem can be demonstrated clear back to 8.0, but the fix would be too invasive in earlier branches; not to mention that people may be depending on the subtly-incorrect behavior. The 8.4 series is new enough that fixing this probably won't cause complaints, but it might in older branches. Also, 8.4 shows the incorrect behavior in more cases than older branches do, because it is able to flatten subqueries in more cases.
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‎src/backend/optimizer/path/allpaths.c

Lines changed: 3 additions & 2 deletions
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@@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.184 2009/07/06 18:26:30 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.185 2009/09/02 17:52:24 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
@@ -1254,7 +1254,8 @@ subquery_push_qual(Query *subquery, RangeTblEntry *rte, Index rti, Node *qual)
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*/
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qual = ResolveNew(qual, rti, 0, rte,
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subquery->targetList,
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CMD_SELECT, 0);
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CMD_SELECT, 0,
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&subquery->hasSubLinks);
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/*
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* Now attach the qual to the proper place: normally WHERE, but if the

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