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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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2
src/backend/optimizer/plan/analyzejoins.c
View File

@ -396,6 +396,8 @@ remove_rel_from_query(PlannerInfo *root, int relid, Relids joinrelids)
phinfo->ph_eval_at = bms_add_member(phinfo->ph_eval_at, relid);
phinfo->ph_needed = bms_del_member(phinfo->ph_needed, relid);
/* ph_may_need probably isn't used after this, but fix it anyway */
phinfo->ph_may_need = bms_del_member(phinfo->ph_may_need, relid);
}
/*

37
src/backend/optimizer/plan/initsplan.c
View File

@ -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

23
src/backend/optimizer/plan/planmain.c
View File

@ -180,14 +180,19 @@ query_planner(PlannerInfo *root, List *tlist,
add_base_rels_to_query(root, (Node *) parse->jointree);
/*
* Examine the targetlist and qualifications, adding entries to baserel
* targetlists for all referenced Vars. Restrict and join clauses are
* added to appropriate lists belonging to the mentioned relations. We
* also build EquivalenceClasses for provably equivalent expressions, and
* form a target joinlist for make_one_rel() to work from.
* Examine the targetlist and join tree, adding entries to baserel
* targetlists for all referenced Vars, and generating PlaceHolderInfo
* entries for all referenced PlaceHolderVars. Restrict and join clauses
* are added to appropriate lists belonging to the mentioned relations.
* We also build EquivalenceClasses for provably equivalent expressions.
* The SpecialJoinInfo list is also built to hold information about join
* order restrictions. Finally, we form a target joinlist for
* make_one_rel() to work from.
*/
build_base_rel_tlists(root, tlist);
find_placeholders_in_jointree(root);
joinlist = deconstruct_jointree(root);
/*
@ -218,10 +223,12 @@ query_planner(PlannerInfo *root, List *tlist,
/*
* Examine any "placeholder" expressions generated during subquery pullup.
* Make sure that we know what level to evaluate them at, and that the
* Vars they need are marked as needed.
* Make sure that the Vars they need are marked as needed at the relevant
* join level. This must be done before join removal because it might
* cause Vars or placeholders to be needed above a join when they weren't
* so marked before.
*/
fix_placeholder_eval_levels(root);
fix_placeholder_input_needed_levels(root);
/*
* Remove any useless outer joins. Ideally this would be done during

238
src/backend/optimizer/util/placeholder.c
View File

@ -22,6 +22,11 @@
#include "optimizer/var.h"
#include "utils/lsyscache.h"
/* Local functions */
static Relids find_placeholders_recurse(PlannerInfo *root, Node *jtnode);
static void find_placeholders_in_qual(PlannerInfo *root, Node *qual,
Relids relids);
/*
* make_placeholder_expr
@ -47,6 +52,12 @@ make_placeholder_expr(PlannerInfo *root, Expr *expr, Relids phrels)
* find_placeholder_info
* Fetch the PlaceHolderInfo for the given PHV; create it if not found
*
* This is separate from make_placeholder_expr because subquery pullup has
* to make PlaceHolderVars for expressions that might not be used at all in
* the upper query, or might not remain after const-expression simplification.
* We build PlaceHolderInfos only for PHVs that are still present in the
* simplified query passed to query_planner().
*
* Note: this should only be called after query_planner() has started.
*/
PlaceHolderInfo *
@ -71,8 +82,9 @@ find_placeholder_info(PlannerInfo *root, PlaceHolderVar *phv)
phinfo->phid = phv->phid;
phinfo->ph_var = copyObject(phv);
phinfo->ph_eval_at = pull_varnos((Node *) phv);
/* ph_eval_at may change later, see fix_placeholder_eval_levels */
/* ph_eval_at may change later, see update_placeholder_eval_levels */
phinfo->ph_needed = NULL; /* initially it's unused */
phinfo->ph_may_need = NULL;
/* for the moment, estimate width using just the datatype info */
phinfo->ph_width = get_typavgwidth(exprType((Node *) phv->phexpr),
exprTypmod((Node *) phv->phexpr));
@ -83,21 +95,168 @@ find_placeholder_info(PlannerInfo *root, PlaceHolderVar *phv)
}
/*
* fix_placeholder_eval_levels
* find_placeholders_in_jointree
* Search the jointree for PlaceHolderVars, and build PlaceHolderInfos
*
* We don't need to look at the targetlist because build_base_rel_tlists()
* will already have made entries for any PHVs in the tlist.
*/
void
find_placeholders_in_jointree(PlannerInfo *root)
{
/* We need do nothing if the query contains no PlaceHolderVars */
if (root->glob->lastPHId != 0)
{
/* Start recursion at top of jointree */
Assert(root->parse->jointree != NULL &&
IsA(root->parse->jointree, FromExpr));
(void) find_placeholders_recurse(root, (Node *) root->parse->jointree);
}
}
/*
* find_placeholders_recurse
* One recursion level of find_placeholders_in_jointree.
*
* jtnode is the current jointree node to examine.
*
* The result is the set of base Relids contained in or below jtnode.
* This is just an internal convenience, it's not used at the top level.
*/
static Relids
find_placeholders_recurse(PlannerInfo *root, Node *jtnode)
{
Relids jtrelids;
if (jtnode == NULL)
return NULL;
if (IsA(jtnode, RangeTblRef))
{
int varno = ((RangeTblRef *) jtnode)->rtindex;
/* No quals to deal with, just return correct result */
jtrelids = bms_make_singleton(varno);
}
else if (IsA(jtnode, FromExpr))
{
FromExpr *f = (FromExpr *) jtnode;
ListCell *l;
/*
* First, recurse to handle child joins, and form their relid set.
*/
jtrelids = NULL;
foreach(l, f->fromlist)
{
Relids sub_relids;
sub_relids = find_placeholders_recurse(root, lfirst(l));
jtrelids = bms_join(jtrelids, sub_relids);
}
/*
* Now process the top-level quals.
*/
find_placeholders_in_qual(root, f->quals, jtrelids);
}
else if (IsA(jtnode, JoinExpr))
{
JoinExpr *j = (JoinExpr *) jtnode;
Relids leftids,
rightids;
/*
* First, recurse to handle child joins, and form their relid set.
*/
leftids = find_placeholders_recurse(root, j->larg);
rightids = find_placeholders_recurse(root, j->rarg);
jtrelids = bms_join(leftids, rightids);
/* Process the qual clauses */
find_placeholders_in_qual(root, j->quals, jtrelids);
}
else
{
elog(ERROR, "unrecognized node type: %d",
(int) nodeTag(jtnode));
jtrelids = NULL; /* keep compiler quiet */
}
return jtrelids;
}
/*
* find_placeholders_in_qual
* Process a qual clause for find_placeholders_in_jointree.
*
* relids is the syntactic join level to mark as the "maybe needed" level
* for each PlaceHolderVar found in the qual clause.
*/
static void
find_placeholders_in_qual(PlannerInfo *root, Node *qual, Relids relids)
{
List *vars;
ListCell *vl;
/*
* pull_var_clause does more than we need here, but it'll do and it's
* convenient to use.
*/
vars = pull_var_clause(qual, PVC_INCLUDE_PLACEHOLDERS);
foreach(vl, vars)
{
PlaceHolderVar *phv = (PlaceHolderVar *) lfirst(vl);
PlaceHolderInfo *phinfo;
/* Ignore any plain Vars */
if (!IsA(phv, PlaceHolderVar))
continue;
/* Create a PlaceHolderInfo entry if there's not one already */
phinfo = find_placeholder_info(root, phv);
/* Mark the PHV as possibly needed at the qual's syntactic level */
phinfo->ph_may_need = bms_add_members(phinfo->ph_may_need, relids);
/*
* This is a bit tricky: the PHV's contained expression may contain
* other, lower-level PHVs. We need to get those into the
* PlaceHolderInfo list, but they aren't going to be needed where the
* outer PHV is referenced. Rather, they'll be needed where the outer
* PHV is evaluated. We can estimate that (conservatively) as the
* syntactic location of the PHV's expression. Recurse to take care
* of any such PHVs.
*/
find_placeholders_in_qual(root, (Node *) phv->phexpr, phv->phrels);
}
list_free(vars);
}
/*
* update_placeholder_eval_levels
* Adjust the target evaluation levels for placeholders
*
* The initial eval_at level set by find_placeholder_info was the set of
* rels used in the placeholder's expression (or the whole subselect if
* the expr is variable-free). If the subselect contains any outer joins
* that can null any of those rels, we must delay evaluation to above those
* joins.
* rels used in the placeholder's expression (or the whole subselect below
* the placeholder's syntactic location, if the expr is variable-free).
* If the subselect contains any outer joins that can null any of those rels,
* we must delay evaluation to above those joins.
*
* We repeat this operation each time we add another outer join to
* root->join_info_list. It's somewhat annoying to have to do that, but
* since we don't have very much information on the placeholders' locations,
* it's hard to avoid. Each placeholder's eval_at level must be correct
* by the time it starts to figure in outer-join delay decisions for higher
* outer joins.
*
* In future we might want to put additional policy/heuristics here to
* try to determine an optimal evaluation level. The current rules will
* result in evaluation at the lowest possible level.
* result in evaluation at the lowest possible level. However, pushing a
* placeholder eval up the tree is likely to further constrain evaluation
* order for outer joins, so it could easily be counterproductive; and we
* don't have enough information at this point to make an intelligent choice.
*/
void
fix_placeholder_eval_levels(PlannerInfo *root)
update_placeholder_eval_levels(PlannerInfo *root, SpecialJoinInfo *new_sjinfo)
{
ListCell *lc1;
@ -105,16 +264,27 @@ fix_placeholder_eval_levels(PlannerInfo *root)
{
PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(lc1);
Relids syn_level = phinfo->ph_var->phrels;
Relids eval_at = phinfo->ph_eval_at;
Relids eval_at;
bool found_some;
ListCell *lc2;
/*
* We don't need to do any work on this placeholder unless the
* newly-added outer join is syntactically beneath its location.
*/
if (!bms_is_subset(new_sjinfo->syn_lefthand, syn_level) ||
!bms_is_subset(new_sjinfo->syn_righthand, syn_level))
continue;
/*
* Check for delays due to lower outer joins. This is the same logic
* as in check_outerjoin_delay in initsplan.c, except that we don't
* want to modify the delay_upper_joins flags; that was all handled
* already during distribute_qual_to_rels.
* have anything to do with the delay_upper_joins flags; delay of
* upper outer joins will be handled later, based on the eval_at
* values we compute now.
*/
eval_at = phinfo->ph_eval_at;
do
{
found_some = false;
@ -122,7 +292,7 @@ fix_placeholder_eval_levels(PlannerInfo *root)
{
SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(lc2);
/* disregard joins not within the expr's sub-select */
/* disregard joins not within the PHV's sub-select */
if (!bms_is_subset(sjinfo->syn_lefthand, syn_level) ||
!bms_is_subset(sjinfo->syn_righthand, syn_level))
continue;
@ -149,16 +319,32 @@ fix_placeholder_eval_levels(PlannerInfo *root)
} while (found_some);
phinfo->ph_eval_at = eval_at;
}
}
/*
* Now that we know where to evaluate the placeholder, make sure that
* any vars or placeholders it uses will be available at that join
* level. NOTE: this could cause more PlaceHolderInfos to be added to
* placeholder_list. That is okay because we'll process them before
* falling out of the foreach loop. Also, it could cause the
* ph_needed sets of existing list entries to expand, which is also
* okay because this loop doesn't examine those.
*/
/*
* fix_placeholder_input_needed_levels
* Adjust the "needed at" levels for placeholder inputs
*
* This is called after we've finished determining the eval_at levels for
* all placeholders. We need to make sure that all vars and placeholders
* needed to evaluate each placeholder will be available at the join level
* where the evaluation will be done. Note that this loop can have
* side-effects on the ph_needed sets of other PlaceHolderInfos; that's okay
* because we don't examine ph_needed here, so there are no ordering issues
* to worry about.
*/
void
fix_placeholder_input_needed_levels(PlannerInfo *root)
{
ListCell *lc;
foreach(lc, root->placeholder_list)
{
PlaceHolderInfo *phinfo = (PlaceHolderInfo *) lfirst(lc);
Relids eval_at = phinfo->ph_eval_at;
/* No work unless it'll be evaluated above baserel level */
if (bms_membership(eval_at) == BMS_MULTIPLE)
{
List *vars = pull_var_clause((Node *) phinfo->ph_var->phexpr,
@ -175,11 +361,11 @@ fix_placeholder_eval_levels(PlannerInfo *root)
* Add any required PlaceHolderVars to base rels' targetlists.
*
* If any placeholder can be computed at a base rel and is needed above it,
* add it to that rel's targetlist. We have to do this separately from
* fix_placeholder_eval_levels() because join removal happens in between,
* and can change the ph_eval_at sets. There is essentially the same logic
* in add_placeholders_to_joinrel, but we can't do that part until joinrels
* are formed.
* add it to that rel's targetlist. This might look like it could be merged
* with fix_placeholder_input_needed_levels, but it must be separate because
* join removal happens in between, and can change the ph_eval_at sets. There
* is essentially the same logic in add_placeholders_to_joinrel, but we can't
* do that part until joinrels are formed.
*/
void
add_placeholders_to_base_rels(PlannerInfo *root)