Обсуждение: index scan through a subquery

Why should these queries have different plans?


create table foo (a int PRIMARY KEY);

Q1: explain select max(a) from foo

> Result  (cost=0.04..0.05 rows=1 width=0)
>  InitPlan
>    ->  Limit  (cost=0.00..0.04 rows=1 width=4)
>          ->  Index Scan Backward using foo_pkey on foo
>  (cost=0.00..76.10 rows=2140 width=4)
>                Filter: (a IS NOT NULL)

Q2: explain select max(a) from (select * from foo) as f

> Aggregate  (cost=36.75..36.76 rows=1 width=4)
>  ->  Seq Scan on foo  (cost=0.00..31.40 rows=2140 width=4)


I need the lovely index scan, but my table is hidden behind a view, and
all I get is the ugly sequential scan.  Any ideas on how to convince the
optimizer to unfold the subquery properly?

Bill

Bill Howe <howew@stccmop.org> writes:
> I need the lovely index scan, but my table is hidden behind a view, and
> all I get is the ugly sequential scan.  Any ideas on how to convince the
> optimizer to unfold the subquery properly?

You should provide some context in this sort of gripe, like which PG
version you're using.  But I'm going to guess that it's 8.2.x, because
8.1.x gets it right :-(.  Try the attached.

            regards, tom lane

Index: planagg.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/optimizer/plan/planagg.c,v
retrieving revision 1.25
diff -c -r1.25 planagg.c
*** planagg.c    9 Jan 2007 02:14:13 -0000    1.25
--- planagg.c    6 Feb 2007 06:30:23 -0000
***************
*** 70,75 ****
--- 70,76 ----
  optimize_minmax_aggregates(PlannerInfo *root, List *tlist, Path *best_path)
  {
      Query       *parse = root->parse;
+     FromExpr   *jtnode;
      RangeTblRef *rtr;
      RangeTblEntry *rte;
      RelOptInfo *rel;
***************
*** 102,115 ****
       * We also restrict the query to reference exactly one table, since join
       * conditions can't be handled reasonably.  (We could perhaps handle a
       * query containing cartesian-product joins, but it hardly seems worth the
!      * trouble.)
       */
!     Assert(parse->jointree != NULL && IsA(parse->jointree, FromExpr));
!     if (list_length(parse->jointree->fromlist) != 1)
!         return NULL;
!     rtr = (RangeTblRef *) linitial(parse->jointree->fromlist);
!     if (!IsA(rtr, RangeTblRef))
          return NULL;
      rte = rt_fetch(rtr->rtindex, parse->rtable);
      if (rte->rtekind != RTE_RELATION || rte->inh)
          return NULL;
--- 103,121 ----
       * We also restrict the query to reference exactly one table, since join
       * conditions can't be handled reasonably.  (We could perhaps handle a
       * query containing cartesian-product joins, but it hardly seems worth the
!      * trouble.)  However, the single real table could be buried in several
!      * levels of FromExpr.
       */
!     jtnode = parse->jointree;
!     while (IsA(jtnode, FromExpr))
!     {
!         if (list_length(jtnode->fromlist) != 1)
!             return NULL;
!         jtnode = linitial(jtnode->fromlist);
!     }
!     if (!IsA(jtnode, RangeTblRef))
          return NULL;
+     rtr = (RangeTblRef *) jtnode;
      rte = rt_fetch(rtr->rtindex, parse->rtable);
      if (rte->rtekind != RTE_RELATION || rte->inh)
          return NULL;

Tom Lane wrote:
>> I need the lovely index scan, but my table is hidden behind a view, and
>> all I get is the ugly sequential scan.  Any ideas on how to convince the
>> optimizer to unfold the subquery properly?
>
> You should provide some context in this sort of gripe, like which PG
> version you're using.  But I'm going to guess that it's 8.2.x, because
> 8.1.x gets it right :-(.  Try the attached.

Good guess; I was indeed talking about the "current release" rather than
the "previous release."

Also, apologies for the tone of my post: I was attempting to be jovial,
but in retrospect, I see how it reads as a "gripe,"  which I guess
evoked your frowny-face emoticon.

Thanks for the quick response, elegant fix, and ongoing excellent work!

Cheers,
Bill

> Index: planagg.c
> ===================================================================
> RCS file: /cvsroot/pgsql/src/backend/optimizer/plan/planagg.c,v
> retrieving revision 1.25
> diff -c -r1.25 planagg.c
> *** planagg.c    9 Jan 2007 02:14:13 -0000    1.25
> --- planagg.c    6 Feb 2007 06:30:23 -0000
> ***************
> *** 70,75 ****
> --- 70,76 ----
>   optimize_minmax_aggregates(PlannerInfo *root, List *tlist, Path *best_path)
>   {
>       Query       *parse = root->parse;
> +     FromExpr   *jtnode;
>       RangeTblRef *rtr;
>       RangeTblEntry *rte;
>       RelOptInfo *rel;
> ***************
> *** 102,115 ****
>        * We also restrict the query to reference exactly one table, since join
>        * conditions can't be handled reasonably.  (We could perhaps handle a
>        * query containing cartesian-product joins, but it hardly seems worth the
> !      * trouble.)
>        */
> !     Assert(parse->jointree != NULL && IsA(parse->jointree, FromExpr));
> !     if (list_length(parse->jointree->fromlist) != 1)
> !         return NULL;
> !     rtr = (RangeTblRef *) linitial(parse->jointree->fromlist);
> !     if (!IsA(rtr, RangeTblRef))
>           return NULL;
>       rte = rt_fetch(rtr->rtindex, parse->rtable);
>       if (rte->rtekind != RTE_RELATION || rte->inh)
>           return NULL;
> --- 103,121 ----
>        * We also restrict the query to reference exactly one table, since join
>        * conditions can't be handled reasonably.  (We could perhaps handle a
>        * query containing cartesian-product joins, but it hardly seems worth the
> !      * trouble.)  However, the single real table could be buried in several
> !      * levels of FromExpr.
>        */
> !     jtnode = parse->jointree;
> !     while (IsA(jtnode, FromExpr))
> !     {
> !         if (list_length(jtnode->fromlist) != 1)
> !             return NULL;
> !         jtnode = linitial(jtnode->fromlist);
> !     }
> !     if (!IsA(jtnode, RangeTblRef))
>           return NULL;
> +     rtr = (RangeTblRef *) jtnode;
>       rte = rt_fetch(rtr->rtindex, parse->rtable);
>       if (rte->rtekind != RTE_RELATION || rte->inh)
>           return NULL;
>
> ---------------------------(end of broadcast)---------------------------
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