Re: That EXPLAIN ANALYZE patch still needs work
От | Jim C. Nasby |
---|---|
Тема | Re: That EXPLAIN ANALYZE patch still needs work |
Дата | |
Msg-id | 20060608215328.GD45331@pervasive.com обсуждение исходный текст |
Ответ на | Re: That EXPLAIN ANALYZE patch still needs work (Tom Lane <tgl@sss.pgh.pa.us>) |
Список | pgsql-hackers |
On Thu, Jun 08, 2006 at 05:32:36PM -0400, Tom Lane wrote: > "Jim C. Nasby" <jnasby@pervasive.com> writes: > > The powerbook tests were not very repeatable at 100,000 rows, so I > > bumped up to 1M. The results still aren't very repeatable... > > Hmm, I suspect you pushed it into the realm where it's doing actual > I/O ... which we don't want for this particular test. (Although it's > worth noting that if your query *is* doing actual I/O, the EXPLAIN > overhead certainly ought to be down in the noise.) Hmm... I guess it depends on how smart the OS cache is; the table is 36MB; 4406 pages. But shared buffers is 3000... Bumping shared_buffers up to 5k, there is a bigger difference, but these numbers are still all over the place, so I don't know that they're very trust-worthy. decibel=# explain analyze select count(*) from i; QUERY PLAN -------------------------------------------------------------------------------------------------------------------Aggregate (cost=16905.05..16905.06 rows=1 width=0) (actual time=5398.625..5398.626 rows=1 loops=1) -> Seq Scan on i (cost=0.00..14405.24rows=999924 width=0) (actual time=0.034..3967.927 rows=1000000 loops=1)Total runtime: 5398.871 ms (3 rows) Time: 5400.900 ms decibel=# explain analyze select count(*) from i; QUERY PLAN -------------------------------------------------------------------------------------------------------------------Aggregate (cost=16905.05..16905.06 rows=1 width=0) (actual time=5626.671..5626.671 rows=1 loops=1) -> Seq Scan on i (cost=0.00..14405.24rows=999924 width=0) (actual time=0.035..3875.641 rows=1000000 loops=1)Total runtime: 5626.904 ms (3 rows) Time: 5628.896 ms decibel=# explain analyze select count(*) from i; QUERY PLAN -------------------------------------------------------------------------------------------------------------------Aggregate (cost=16905.05..16905.06 rows=1 width=0) (actual time=5137.549..5137.549 rows=1 loops=1) -> Seq Scan on i (cost=0.00..14405.24rows=999924 width=0) (actual time=0.020..3440.034 rows=1000000 loops=1)Total runtime: 5137.707 ms (3 rows) Time: 5139.178 ms decibel=# select count(*) from i; count ---------1000000 (1 row) Time: 4357.443 ms decibel=# select count(*) from i; count ---------1000000 (1 row) Time: 4251.208 ms decibel=# select count(*) from i; count ---------1000000 (1 row) Time: 4712.912 ms decibel=# select count(*) from i; count ---------1000000 (1 row) Time: 4479.278 ms decibel=# -- Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com Pervasive Software http://pervasive.com work: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461
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