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Avoid O(N^2) cost when pulling up lots of UNION ALL subqueries.

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perform_pullup_replace_vars() knows how to scan the whole parent
query tree when we are replacing Vars during a subquery flattening
operation.  However, for the specific case of flattening a
UNION ALL leaf query, that's mostly wasted work: the only place
where relevant Vars could exist is in the AppendRelInfo that we
just made for this leaf.  Teaching perform_pullup_replace_vars()
to just deal with that and exit is worthwhile because, if we have
N such subqueries to pull up, we were spending O(N^2) work uselessly
mutating the AppendRelInfos for all the other subqueries.

While we're at it, avoid calling substitute_phv_relids if there are no
PlaceHolderVars, and remove an obsolete check of parse->hasSubLinks.

Andrey Lepikhov and Tom Lane

Discussion: https://postgr.es/m/703c09a2-08f3-d2ec-b33d-dbecd62428b8@postgrespro.ru
This commit is contained in:
Tom Lane 2022-12-22 11:02:03 -05:00
parent 5beb7881fb
commit e42e312430
1 changed files with 35 additions and 20 deletions
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55
src/backend/optimizer/prep/prepjointree.c
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@ -128,7 +128,7 @@ static bool find_dependent_phvs_in_jointree(PlannerInfo *root,
Node *node, int varno); Node *node, int varno);
static void substitute_phv_relids(Node *node, static void substitute_phv_relids(Node *node,
int varno, Relids subrelids); int varno, Relids subrelids);
static void fix_append_rel_relids(List *append_rel_list, int varno, static void fix_append_rel_relids(PlannerInfo *root, int varno,
Relids subrelids); Relids subrelids);
static Node *find_jointree_node_for_rel(Node *jtnode, int relid); static Node *find_jointree_node_for_rel(Node *jtnode, int relid);
@ -1232,14 +1232,14 @@ pull_up_simple_subquery(PlannerInfo *root, Node *jtnode, RangeTblEntry *rte,
* already checked that this won't require introducing multiple subrelids * already checked that this won't require introducing multiple subrelids
* into the single-slot AppendRelInfo structs. * into the single-slot AppendRelInfo structs.
*/ */
if (parse->hasSubLinks || root->glob->lastPHId != 0 || if (root->glob->lastPHId != 0 || root->append_rel_list)
root->append_rel_list)
{ {
Relids subrelids; Relids subrelids;
subrelids = get_relids_in_jointree((Node *) subquery->jointree, false); subrelids = get_relids_in_jointree((Node *) subquery->jointree, false);
substitute_phv_relids((Node *) parse, varno, subrelids); if (root->glob->lastPHId != 0)
fix_append_rel_relids(root->append_rel_list, varno, subrelids); substitute_phv_relids((Node *) parse, varno, subrelids);
fix_append_rel_relids(root, varno, subrelids);
} }
/* /*
@ -1418,7 +1418,10 @@ pull_up_union_leaf_queries(Node *setOp, PlannerInfo *root, int parentRTindex,
/* /*
* Recursively apply pull_up_subqueries to the new child RTE. (We * Recursively apply pull_up_subqueries to the new child RTE. (We
* must build the AppendRelInfo first, because this will modify it.) * must build the AppendRelInfo first, because this will modify it;
* indeed, that's the only part of the upper query where Vars
* referencing childRTindex can exist at this point.)
*
* Note that we can pass NULL for containing-join info even if we're * Note that we can pass NULL for containing-join info even if we're
* actually under an outer join, because the child's expressions * actually under an outer join, because the child's expressions
* aren't going to propagate up to the join. Also, we ignore the * aren't going to propagate up to the join. Also, we ignore the
@ -2119,6 +2122,25 @@ perform_pullup_replace_vars(PlannerInfo *root,
Query *parse = root->parse; Query *parse = root->parse;
ListCell *lc; ListCell *lc;
/*
* If we are considering an appendrel child subquery (that is, a UNION ALL
* member query that we're pulling up), then the only part of the upper
* query that could reference the child yet is the translated_vars list of
* the associated AppendRelInfo. Furthermore, we do not need to insert
* PHVs in the AppendRelInfo --- there isn't any outer join between.
*/
if (containing_appendrel)
{
bool save_need_phvs = rvcontext->need_phvs;
rvcontext->need_phvs = false;
containing_appendrel->translated_vars = (List *)
pullup_replace_vars((Node *) containing_appendrel->translated_vars,
rvcontext);
rvcontext->need_phvs = save_need_phvs;
return;
}
/* /*
* Replace all of the top query's references to the subquery's outputs * Replace all of the top query's references to the subquery's outputs
* with copies of the adjusted subtlist items, being careful not to * with copies of the adjusted subtlist items, being careful not to
@ -2172,22 +2194,14 @@ perform_pullup_replace_vars(PlannerInfo *root,
parse->havingQual = pullup_replace_vars(parse->havingQual, rvcontext); parse->havingQual = pullup_replace_vars(parse->havingQual, rvcontext);
/* /*
* Replace references in the translated_vars lists of appendrels. When * Replace references in the translated_vars lists of appendrels.
* pulling up an appendrel member, we do not need PHVs in the list of the
* parent appendrel --- there isn't any outer join between. Elsewhere,
* use PHVs for safety. (This analysis could be made tighter but it seems
* unlikely to be worth much trouble.)
*/ */
foreach(lc, root->append_rel_list) foreach(lc, root->append_rel_list)
{ {
AppendRelInfo *appinfo = (AppendRelInfo *) lfirst(lc); AppendRelInfo *appinfo = (AppendRelInfo *) lfirst(lc);
bool save_need_phvs = rvcontext->need_phvs;
if (appinfo == containing_appendrel)
rvcontext->need_phvs = false;
appinfo->translated_vars = (List *) appinfo->translated_vars = (List *)
pullup_replace_vars((Node *) appinfo->translated_vars, rvcontext); pullup_replace_vars((Node *) appinfo->translated_vars, rvcontext);
rvcontext->need_phvs = save_need_phvs;
} }
/* /*
@ -3358,7 +3372,7 @@ remove_result_refs(PlannerInfo *root, int varno, Node *newjtloc)
subrelids = get_relids_in_jointree(newjtloc, false); subrelids = get_relids_in_jointree(newjtloc, false);
Assert(!bms_is_empty(subrelids)); Assert(!bms_is_empty(subrelids));
substitute_phv_relids((Node *) root->parse, varno, subrelids); substitute_phv_relids((Node *) root->parse, varno, subrelids);
fix_append_rel_relids(root->append_rel_list, varno, subrelids); fix_append_rel_relids(root, varno, subrelids);
} }
/* /*
@ -3577,7 +3591,7 @@ substitute_phv_relids(Node *node, int varno, Relids subrelids)
* We assume we may modify the AppendRelInfo nodes in-place. * We assume we may modify the AppendRelInfo nodes in-place.
*/ */
static void static void
fix_append_rel_relids(List *append_rel_list, int varno, Relids subrelids) fix_append_rel_relids(PlannerInfo *root, int varno, Relids subrelids)
{ {
ListCell *l; ListCell *l;
int subvarno = -1; int subvarno = -1;
@ -3588,7 +3602,7 @@ fix_append_rel_relids(List *append_rel_list, int varno, Relids subrelids)
* AppendRelInfo nodes refer to it. So compute it on first use. Note that * AppendRelInfo nodes refer to it. So compute it on first use. Note that
* bms_singleton_member will complain if set is not singleton. * bms_singleton_member will complain if set is not singleton.
*/ */
foreach(l, append_rel_list) foreach(l, root->append_rel_list)
{ {
AppendRelInfo *appinfo = (AppendRelInfo *) lfirst(l); AppendRelInfo *appinfo = (AppendRelInfo *) lfirst(l);
@ -3603,8 +3617,9 @@ fix_append_rel_relids(List *append_rel_list, int varno, Relids subrelids)
} }
/* Also fix up any PHVs in its translated vars */ /* Also fix up any PHVs in its translated vars */
substitute_phv_relids((Node *) appinfo->translated_vars, if (root->glob->lastPHId != 0)
varno, subrelids); substitute_phv_relids((Node *) appinfo->translated_vars,
varno, subrelids);
} }
} }