Обсуждение: Wrong plan for simple join with index on FK
Hello I test using index on foreign key. I found situation, when planner choose worse plan. create table f1(pk serial primary key); create table f2(fk integer references f1(pk)); insert into f1 select a from generate_series(1,10000) a; insert into f2 select (random()*9999)::int+1 from generate_series(1,140000); vacuum analyze; create index xxx on f2(fk); \timing postgres=> select count(*) from f1 join f2 on pk=fk; count -------- 140000 (1 row) Time: 538,254 ms drop index xxx; postgres=> select count(*) from f1 join f2 on pk=fk; count -------- 140000 (1 row) Time: 311,580 ms Plans: postgres=> explain select count(*) from f1 join f2 on pk=fk; QUERY PLAN -------------------------------------------------------------------------- Aggregate (cost=7788.00..7788.01 rows=1 width=0) -> Hash Join (cost=170.00..7438.00 rows=140000 width=0) HashCond: (f2.fk = f1.pk) -> Seq Scan on f2 (cost=0.00..2018.00 rows=140000 width=4) -> Hash (cost=145.00..145.00rows=10000 width=4) -> Seq Scan on f1 (cost=0.00..145.00 rows=10000 width=4) (6 rows) postgres=> explain select count(*) from f1 join f2 on pk=fk; QUERY PLAN ------------------------------------------------------------------------------------ Aggregate (cost=6631.75..6631.76 rows=1 width=0) -> Merge Join (cost=0.00..6281.75 rows=140000 width=0) MergeCond: (f1.pk = f2.fk) -> Index Scan using f1_pkey on f1 (cost=0.00..187.00 rows=10000 width=4) -> Index Scan using xxx on f2 (cost=0.00..4319.77 rows=140000 width=4) (5 rows) PostgreSQL 8.1, Linux Regards Pavel Stehule _________________________________________________________________ Citite se osamele? Poznejte nekoho vyjmecneho diky Match.com. http://www.msn.cz/
On Tue, May 16, 2006 at 11:52:05AM +0200, Pavel Stehule wrote: > Hello > > I test using index on foreign key. I found situation, when planner choose > worse plan. Can we seen an EXPLAIN ANALYZE output to see where the miscalculation lies. Is it underestimating the cost of the index scan, or overestimating the cost of the hash join. Have a nice day, -- Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to litigate.
>
>Can we seen an EXPLAIN ANALYZE output to see where the miscalculation
>lies. Is it underestimating the cost of the index scan, or
>overestimating the cost of the hash join.
postgres=> explain analyze select count(*) from f1 join f2 on pk=fk;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=6631.75..6631.76 rows=1 width=0) (actual
time=2433.700..2433.703 rows=1 loops=1) -> Merge Join (cost=0.00..6281.75 rows=140000 width=0) (actual
time=0.055..1916.815 rows=140000 loops=1) Merge Cond: (f1.pk = f2.fk) -> Index Scan using f1_pkey on f1
(cost=0.00..187.00rows=10000
width=4) (actual time=0.025..45.635 rows=10000 loops=1) -> Index Scan using xxx on f2 (cost=0.00..4319.77
rows=140000
width=4) (actual time=0.011..812.661 rows=140000 loops=1)
Total runtime: 2433.859 ms
(6 rows)
postgres=> explain analyze select count(*) from f1 join f2 on pk=fk;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=7788.00..7788.01 rows=1 width=0) (actual
time=2216.490..2216.493 rows=1 loops=1) -> Hash Join (cost=170.00..7438.00 rows=140000 width=0) (actual
time=80.296..1712.505 rows=140000 loops=1) Hash Cond: (f2.fk = f1.pk) -> Seq Scan on f2
(cost=0.00..2018.00rows=140000 width=4)
(actual time=0.031..493.614 rows=140000 loops=1) -> Hash (cost=145.00..145.00 rows=10000 width=4) (actual
time=80.201..80.201 rows=10000 loops=1) -> Seq Scan on f1 (cost=0.00..145.00 rows=10000 width=4)
(actual time=0.025..37.587 rows=10000 loops=1)
Total runtime: 2216.730 ms
(7 rows)
Regards
Pavel
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On Tue, May 16, 2006 at 12:54:58PM +0200, Pavel Stehule wrote: > > > >Can we seen an EXPLAIN ANALYZE output to see where the miscalculation > >lies. Is it underestimating the cost of the index scan, or > >overestimating the cost of the hash join. > postgres=> explain analyze select count(*) from f1 join f2 on pk=fk; First query (merge join): Apart from the apparent overestimation of the cost of a full index scan over xxx by about 30%, there seems to be a significant underestimation of the cost of the merge join. > postgres=> explain analyze select count(*) from f1 join f2 on pk=fk; Second query (hash join): Here the estimates seem to be fine, except for an apparent underestimation of the cost of the aggregate. These are all minor abberations though, on the whole the estimates are pretty good. Perhaps you need to tweak the values of random_page_cost and similar variables. Have a ncie day, -- Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to litigate.
> >These are all minor abberations though, on the whole the estimates are >pretty good. Perhaps you need to tweak the values of random_page_cost >and similar variables. > Thank You, It's general problem or only mine? I have "100%" standard current PC. Pavel _________________________________________________________________ Najdete si svou lasku a nove pratele na Match.com. http://www.msn.cz/
> >These are all minor abberations though, on the whole the estimates are > >pretty good. Perhaps you need to tweak the values of random_page_cost > >and similar variables. > > Thank You, It's general problem or only mine? I have "100%" > standard current PC. The default random_page_cost assumes some concurrent activity. If your PC does nothing else concurrently, the performance of a seq scan will be underestimated. Try to do the statement with some concurrent disk load and you will most likely see that the 1. plan is faster. (assuming the tables are not fully cached) Andreas
> > >These are all minor abberations though, on the whole the estimates >are > > >pretty good. Perhaps you need to tweak the values of random_page_cost > > >and similar variables. > > > > Thank You, It's general problem or only mine? I have "100%" > > standard current PC. > >The default random_page_cost assumes some concurrent activity. If your >PC does nothing else concurrently, the performance of a seq scan will >be underestimated. > >Try to do the statement with some concurrent disk load and you will most >likely >see that the 1. plan is faster. (assuming the tables are not fully >cached) > >Andreas ok. I tested it with pgbench and it's true. With -c 50 merge_join is faster. I didn't expect it. Thank You Pavel _________________________________________________________________ Citite se osamele? Poznejte nekoho vyjmecneho diky Match.com. http://www.msn.cz/