This related to a post in the general bugs forum, but I found this forum, and this seems more appropriate. This is my second attempt to post, I believe the first attempt last week did not work, apologies if I'm duplicating.
http://comments.gmane.org/gmane.comp.db.postgresql.bugs/39011 I made have several users encounter performance problems, which all seem to come down to this problem: multiplying selectivity estimates can cause tuple estimates to grow very small very quickly, once the estimator gets to 1 row, the planner may choose plans that are very good ONLY WHEN there is exactly 1 row (maybe even O(N^large)). Unfortunately, these may be the worst plans if the estimate is even slightly off (even just returning 2 or 3 rows versus 1). Using the patch below, I discovered that clamping relation tuple estimates to a number as small as 2 seemed to avoid all the catastrophic query plans. In the scenarios I'm seeing, I have several examples of queries that take >1m to run that should run in <1s. The estimate of 1 row (versus thousands actual) leads the planner to tee up several nest loop joins which causes thousands of table scans. I have been working on a more complete which tracks uniqueness along with selectivity so that optimizer can benefit from knowing when a relation must have 1 (or fewer) tuples, while clamping all other relations to 2 rather than 1. typedef struct { double selectivity; boolean unique; } Selectivity; I am interested in hearing discussion about this problem, and if the community is open to a patch if I continue pursuing the development. Matt FIRST ARTIFACT plan with expensive (80s join) and join estimate of 1 note the first Nested Loop join and 81s join (I gave up trying to post the full explain, because of the 80 char limit) "Sort (cost=7000.04..7000.04 rows=1 width=49) (actual time=81739.426..81740.023 rows=5091 loops=1)" " Sort Key: c.ten DESC" " Sort Method: quicksort Memory: 948kB" " CTE cte" " -> Values Scan on "*VALUES*" (cost=0.00..0.06 rows=5 width=4) (actual time=0.001..0.001 rows=5 loops=1)" " -> Nested Loop (cost=1.36..6999.97 rows=1 width=49) (actual time=0.059..81725.475 rows=5091 loops=1)" "Planning time: 1.912 ms" "Execution time: 81740.328 ms" SECOND ARTIFACT force join row estimate to be minimun of 2 query completes very quickly "Sort (cost=7000.06..7000.06 rows=2 width=49) (actual time=84.610..85.192 rows=5142 loops=1)" " Sort Key: c.ten DESC" " Sort Method: quicksort Memory: 956kB" " CTE cte" " -> Values Scan on "*VALUES*" (cost=0.00..0.06 rows=5 width=4) (actual time=0.002..0.003 rows=5 loops=1)" " -> Hash Join (cost=2518.99..6999.98 rows=2 width=49) (actual time=17.629..82.886 rows=5142 loops=1)" "Planning time: 2.982 ms" "Execution time: 85.514 ms" THIRD ARTIFACT patch I used to make experimenting easier w/o recompiling index 1b61fd9..444703c 100644 --- a/src/backend/optimizer/path/costsize.c +++ b/src/backend/optimizer/path/costsize.c @@ -68,6 +68,12 @@ *------------------------------------------------------------------------- */ + + +/* These parameters are set by GUC */ +int join_row_estimate_clamp=1; + + #include "postgres.h" #ifdef _MSC_VER @@ -175,6 +181,17 @@ clamp_row_est(double nrows) } +double +clamp_join_row_est(double nrows) +{ + nrows = clamp_row_est(nrows); + if (nrows >= (double)join_row_estimate_clamp) + return nrows; + return (double)join_row_estimate_clamp; +} + + + /* * cost_seqscan * Determines and returns the cost of scanning a relation sequentially. @@ -3886,7 +3903,7 @@ calc_joinrel_size_estimate(PlannerInfo *root, break; } - return clamp_row_est(nrows); + return clamp_join_row_est(nrows); } /* diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c index 71090f2..fabb8ac 100644 --- a/src/backend/utils/misc/guc.c +++ b/src/backend/utils/misc/guc.c @@ -2664,6 +2664,16 @@ static struct config_int ConfigureNamesInt[] = NULL, NULL, NULL }, + { + {"join_row_estimate_clamp", PGC_USERSET, QUERY_TUNING_OTHER, + gettext_noop("Set the minimum estimated size of a join result."), + NULL + }, + &join_row_estimate_clamp, + 1, 1, 10000, + NULL, NULL, NULL + }, + /* End-of-list marker */ { {NULL, 0, 0, NULL, NULL}, NULL, 0, 0, 0, NULL, NULL, NULL diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h index 25a7303..0161c4b 100644 --- a/src/include/optimizer/cost.h +++ b/src/include/optimizer/cost.h @@ -67,8 +67,10 @@ extern bool enable_material; extern bool enable_mergejoin; extern bool enable_hashjoin; extern int constraint_exclusion; +extern int join_row_estimate_clamp; extern double clamp_row_est(double nrows); +extern double clamp_join_row_est(double nrows); extern double index_pages_fetched(double tuples_fetched, BlockNumber pages, double index_pages, PlannerInfo *root); extern void cost_seqscan(Path *path, PlannerInfo *root, RelOptInfo *baserel,