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Fix a couple of oversights associated with the "physical tlist" optimization:

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we had several code paths where a physical tlist could be used for the input
to a Sort node, which is a dumb idea because any unneeded table columns will
increase the volume of data the sort has to push around.

(Unfortunately the easy-looking fix of calling disuse_physical_tlist during
make_sort_xxx doesn't work because in most cases we're already committed to
the current input tlist --- it's been marked with sort column numbers, or
we've built grouping column numbers using it, etc.  The tlist has to be
selected properly at the calling level before we start constructing sort-col
information.  This is easy enough to do, we were just failing to take the
point into consideration.)

Back-patch to 8.3.  I believe the problem probably exists clear back to 7.4
when the physical tlist optimization was added, but I'm afraid to back-patch
further than 8.3 without a great deal more study than I want to put into it.
The code in this area has drifted a lot over time.  The real-world importance
of these code paths is uncertain anyway --- I think in many cases we'd
probably prefer hash-based methods.
This commit is contained in:
Tom Lane
2008-04-17 21:22:14 +00:00
parent c5c7ba1b10
commit 25e46a504b
3 changed files with 27 additions and 12 deletions
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21
src/backend/optimizer/plan/planner.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.231 2008/04/01 00:48:33 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.232 2008/04/17 21:22:14 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -957,9 +957,23 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
* Normal case --- create a plan according to query_planner's
* results.
*/
bool need_sort_for_grouping = false;
result_plan = create_plan(root, best_path);
current_pathkeys = best_path->pathkeys;
/* Detect if we'll need an explicit sort for grouping */
if (parse->groupClause && !use_hashed_grouping &&
!pathkeys_contained_in(group_pathkeys, current_pathkeys))
{
need_sort_for_grouping = true;
/*
* Always override query_planner's tlist, so that we don't
* sort useless data from a "physical" tlist.
*/
need_tlist_eval = true;
}
/*
* create_plan() returns a plan with just a "flat" tlist of
* required Vars. Usually we need to insert the sub_tlist as the
@ -1054,8 +1068,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
if (parse->groupClause)
{
if (!pathkeys_contained_in(group_pathkeys,
current_pathkeys))
if (need_sort_for_grouping)
{
result_plan = (Plan *)
make_sort_from_groupcols(root,
@ -1098,7 +1111,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
* Add an explicit sort if we couldn't make the path come out
* the way the GROUP node needs it.
*/
if (!pathkeys_contained_in(group_pathkeys, current_pathkeys))
if (need_sort_for_grouping)
{
result_plan = (Plan *)
make_sort_from_groupcols(root,