Re: That EXPLAIN ANALYZE patch still needs work
От | Simon Riggs |
---|---|
Тема | Re: That EXPLAIN ANALYZE patch still needs work |
Дата | |
Msg-id | 1149799677.9225.15.camel@localhost.localdomain обсуждение исходный текст |
Ответ на | Re: That EXPLAIN ANALYZE patch still needs work (Tom Lane <tgl@sss.pgh.pa.us>) |
Ответы |
Re: That EXPLAIN ANALYZE patch still needs work
Re: That EXPLAIN ANALYZE patch still needs work |
Список | pgsql-hackers |
On Thu, 2006-06-08 at 12:56 -0400, Tom Lane wrote: > Simon Riggs <simon@2ndquadrant.com> writes: > > So far we have myself, Kevin, Martijn and Luke all saying there is a > > distortion or a massive overhead caused by EXPLAIN ANALYZE. > > http://archives.postgresql.org/pgsql-hackers/2006-03/msg00954.php > > http://archives.postgresql.org/pgsql-patches/2006-05/msg00168.php > > Given that we're seeing diametrically opposed results on the same OS > (FC5) and similar (at least all Intel) hardware, I think the prudent > thing is to find out what's really going on before leaping in with > proposed solutions. As the person who's *not* seeing the problem, > I'm not in a position to do that investigation... That seems reasonable. I've cut a patch to remove timing from the EA results. Output shown here: postgres=# set explain_analyze_timing = on; SET Time: 0.673 ms postgres=# explain analyze select count(*) from accounts; QUERY PLAN ------------------------------------------------------------------------------------------------------------------------ Aggregate (cost=2890.00..2890.01 rows=1 width=0) (actual time=0.000..690.780 rows=1 loops=1) -> Seq Scan on accounts (cost=0.00..2640.00 rows=100000 width=0) (actual time=0.000..2672.562 rows=100000 loops=1) Total runtime: 734.474 ms (3 rows) Time: 891.822 ms postgres=# set explain_analyze_timing = off; SET Time: 0.480 ms postgres=# explain analyze select count(*) from accounts; QUERY PLAN -------------------------------------------------------------------------------------------- Aggregate (cost=2890.00..2890.01 rows=1 width=0) (rows=1 loops=1) -> Seq Scan on accounts (cost=0.00..2640.00 rows=100000 width=0) (rows=100000 loops=1) Total runtime: 133.674 ms (3 rows) Time: 134.565 ms postgres=# select count(*) from accounts; count -------- 100000 (1 row) Time: 130.528 ms So the timing is clearly responsible for the additional time I'm personally experiencing and very likely to be that for others also. As to why that should be the case, I'm not sure. The timing overhead seems fairly constant on particular hardware/OS, just different for each. -- Simon Riggs EnterpriseDB http://www.enterprisedb.com
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