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Allow subquery pullup to wrap a PlaceHolderVar in another one.

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The code for wrapping subquery output expressions in PlaceHolderVars
believed that if the expression already was a PlaceHolderVar, it was
never necessary to wrap that in another one.  That's wrong if the
expression is underneath an outer join and involves a lateral
reference to outside that scope: failing to add an additional PHV
risks evaluating the expression at the wrong place and hence not
forcing it to null when the outer join should do so.  This is an
oversight in commit 9e7e29c75, which added logic to forcibly wrap
lateral-reference Vars in PlaceHolderVars, but didn't see that the
adjacent case for PlaceHolderVars needed the same treatment.

The test case we have for this doesn't fail before 4be058fe9, but now
that I see the problem I wonder if it is possible to demonstrate
related errors before that.  That's moot though, since all such
branches are out of support.

Per bug #18284 from Holger Reise.  Back-patch to all supported
branches.

Discussion: https://postgr.es/m/18284-47505a20c23647f8@postgresql.org
This commit is contained in:
Tom Lane
2024-01-11 15:28:13 -05:00
parent af36f1993d
commit 80bece312c
3 changed files with 46 additions and 2 deletions
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9
src/backend/optimizer/prep/prepjointree.c
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@@ -2435,8 +2435,13 @@ pullup_replace_vars_callback(Var *var,
else if (newnode && IsA(newnode, PlaceHolderVar) &&
((PlaceHolderVar *) newnode)->phlevelsup == 0)
{
/* No need to wrap a PlaceHolderVar with another one, either */
wrap = false;
/* The same rules apply for a PlaceHolderVar */
if (rcon->target_rte->lateral &&
!bms_is_subset(((PlaceHolderVar *) newnode)->phrels,
rcon->relids))
wrap = true;
else
wrap = false;
}
else
{