Re: [PERFORM] Planner doesn't look at LIMIT? - Mailing list pgsql-hackers

I wrote:
> Dawid Kuroczko <qnex42@gmail.com> writes:
>> qnex=# EXPLAIN SELECT * FROM log NATURAL JOIN useragents LIMIT 1;

>> Limit  (cost=15912.20..15912.31 rows=1 width=272)
>> ->  Hash Join  (cost=15912.20..5328368.96 rows=47044336 width=272)

>> If I set enable_hashjoin=false:

>> qnex=# EXPLAIN ANALYZE SELECT * FROM log NATURAL LEFT JOIN useragents LIMIT 1;

>> Limit  (cost=0.00..3.07 rows=1 width=272) (actual time=74.214..74.216
>> rows=1 loops=1)
>> ->  Nested Loop Left Join  (cost=0.00..144295895.01 rows=47044336
>> width=272) (actual time=74.204..74.204 rows=1 loops=1)

> This is quite strange.  The nestloop plan definitely should be preferred
> in the context of the LIMIT, considering that it has far lower estimated
> cost.  And it is preferred in simple tests for me.

After a suitable period of contemplating my navel, I figured out
what is going on here: the total costs involved are large enough that
the still-fairly-high startup cost of the hash is disregarded by
compare_fuzzy_path_costs(), and so the nestloop is discarded as not
having any significant potential advantage in startup time.

I think that this refutes the original scheme of using the same fuzz
factor for both startup and total cost comparisons, and therefore
propose the attached patch.

Comments?

            regards, tom lane

*** src/backend/optimizer/util/pathnode.c.orig    Fri Jul 15 13:09:25 2005
--- src/backend/optimizer/util/pathnode.c    Fri Jul 22 12:08:25 2005
***************
*** 98,157 ****
  static int
  compare_fuzzy_path_costs(Path *path1, Path *path2, CostSelector criterion)
  {
-     Cost        fuzz;
-
      /*
!      * The fuzz factor is set at one percent of the smaller total_cost,
!      * but not less than 0.01 cost units (just in case total cost is
!      * zero).
       *
       * XXX does this percentage need to be user-configurable?
       */
-     fuzz = Min(path1->total_cost, path2->total_cost) * 0.01;
-     fuzz = Max(fuzz, 0.01);
-
      if (criterion == STARTUP_COST)
      {
!         if (Abs(path1->startup_cost - path2->startup_cost) > fuzz)
!         {
!             if (path1->startup_cost < path2->startup_cost)
!                 return -1;
!             else
!                 return +1;
!         }

          /*
           * If paths have the same startup cost (not at all unlikely),
           * order them by total cost.
           */
!         if (Abs(path1->total_cost - path2->total_cost) > fuzz)
!         {
!             if (path1->total_cost < path2->total_cost)
!                 return -1;
!             else
!                 return +1;
!         }
      }
      else
      {
!         if (Abs(path1->total_cost - path2->total_cost) > fuzz)
!         {
!             if (path1->total_cost < path2->total_cost)
!                 return -1;
!             else
!                 return +1;
!         }

          /*
           * If paths have the same total cost, order them by startup cost.
           */
!         if (Abs(path1->startup_cost - path2->startup_cost) > fuzz)
!         {
!             if (path1->startup_cost < path2->startup_cost)
!                 return -1;
!             else
!                 return +1;
!         }
      }
      return 0;
  }
--- 98,138 ----
  static int
  compare_fuzzy_path_costs(Path *path1, Path *path2, CostSelector criterion)
  {
      /*
!      * We use a fuzz factor of 1% of the smaller cost.
       *
       * XXX does this percentage need to be user-configurable?
       */
      if (criterion == STARTUP_COST)
      {
!         if (path1->startup_cost > path2->startup_cost * 1.01)
!             return +1;
!         if (path2->startup_cost > path1->startup_cost * 1.01)
!             return -1;

          /*
           * If paths have the same startup cost (not at all unlikely),
           * order them by total cost.
           */
!         if (path1->total_cost > path2->total_cost * 1.01)
!             return +1;
!         if (path2->total_cost > path1->total_cost * 1.01)
!             return -1;
      }
      else
      {
!         if (path1->total_cost > path2->total_cost * 1.01)
!             return +1;
!         if (path2->total_cost > path1->total_cost * 1.01)
!             return -1;

          /*
           * If paths have the same total cost, order them by startup cost.
           */
!         if (path1->startup_cost > path2->startup_cost * 1.01)
!             return +1;
!         if (path2->startup_cost > path1->startup_cost * 1.01)
!             return -1;
      }
      return 0;
  }