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Fix PlaceHolderVar mechanism's interaction with outer joins.

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The point of a PlaceHolderVar is to allow a non-strict expression to be
evaluated below an outer join, after which its value bubbles up like a Var
and can be forced to NULL when the outer join's semantics require that.
However, there was a serious design oversight in that, namely that we
didn't ensure that there was actually a correct place in the plan tree
to evaluate the placeholder :-(.  It may be necessary to delay evaluation
of an outer join to ensure that a placeholder that should be evaluated
below the join can be evaluated there.  Per recent bug report from Kirill
Simonov.

Back-patch to 8.4 where the PlaceHolderVar mechanism was introduced.
This commit is contained in:
Tom Lane
2010-09-28 12:08:56 -04:00
parent 9c5f4f6cb5
commit eb22950510
11 changed files with 374 additions and 36 deletions
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37
src/backend/optimizer/plan/initsplan.c
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@@ -187,6 +187,13 @@ add_vars_to_targetlist(PlannerInfo *root, List *vars, Relids where_needed)
phinfo->ph_needed = bms_add_members(phinfo->ph_needed,
where_needed);
/*
* Update ph_may_need too. This is currently only necessary
* when being called from build_base_rel_tlists, but we may as
* well do it always.
*/
phinfo->ph_may_need = bms_add_members(phinfo->ph_may_need,
where_needed);
}
else
elog(ERROR, "unrecognized node type: %d", (int) nodeTag(node));
@@ -465,7 +472,11 @@ deconstruct_recurse(PlannerInfo *root, Node *jtnode, bool below_outer_join,
/* Now we can add the SpecialJoinInfo to join_info_list */
if (sjinfo)
{
root->join_info_list = lappend(root->join_info_list, sjinfo);
/* Each time we do that, recheck placeholder eval levels */
update_placeholder_eval_levels(root, sjinfo);
}
/*
* Finally, compute the output joinlist. We fold subproblems together
@@ -687,6 +698,32 @@ make_outerjoininfo(PlannerInfo *root,
}
}
/*
* Examine PlaceHolderVars. If a PHV is supposed to be evaluated within
* this join's nullable side, and it may get used above this join, then
* ensure that min_righthand contains the full eval_at set of the PHV.
* This ensures that the PHV actually can be evaluated within the RHS.
* Note that this works only because we should already have determined
* the final eval_at level for any PHV syntactically within this join.
*/
foreach(l, root->placeholder_list)
{
PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(l);
Relids ph_syn_level = phinfo->ph_var->phrels;
/* Ignore placeholder if it didn't syntactically come from RHS */
if (!bms_is_subset(ph_syn_level, right_rels))
continue;
/* We can also ignore it if it's certainly not used above this join */
/* XXX this test is probably overly conservative */
if (bms_is_subset(phinfo->ph_may_need, min_righthand))
continue;
/* Else, prevent join from being formed before we eval the PHV */
min_righthand = bms_add_members(min_righthand, phinfo->ph_eval_at);
}
/*
* If we found nothing to put in min_lefthand, punt and make it the full
* LHS, to avoid having an empty min_lefthand which will confuse later