pg.ddx.io pgsql-performance@postgresql.org mailing list archive
help / color / mirror / Atom feedR: DELETE queries slow down
2+ messages / 2 participants
[nested] [flat]
* R: DELETE queries slow down
@ 2007-09-18 07:17 Galantucci Giovanni <Giovanni.Galantucci@italtel.it>
2007-09-18 16:18 ` Re: R: DELETE queries slow down Bill Moran <wmoran@collaborativefusion.com>
0 siblings, 1 reply; 2+ messages in thread
From: Galantucci Giovanni @ 2007-09-18 07:17 UTC (permalink / raw)
To: Gregory Stark <stark@enterprisedb.com>; Heikki Linnakangas <heikki@enterprisedb.com>; +Cc: pgsql-performance
I perform simple INSERT and simple where-clause DELETE.
I also force a commit after every DELETE.
My two tables are about these:
TABLE_A
Column_1 | column2 | .......
TABLE_B
Column_1B foreign key references TABLE_A(column_1) on delete cascade | .........
Every row in TABLE_B is also present in TABLE_A, but the contrary is not true.
After hours in which I insert and delete only on TABLE_A (everything ok), I start inserting also on TABLE_B, exploiting the constrain on column_1B. After the first DELETE I perform on both tables, each following DELETE lasts for minutes, with cpu usage on 99,9%.
I tried also to perform a VACUUM after each DELETE, but had no benefits.
Even the EXPLAIN ANALYZE of the DELETE shows no changes with respect to the previous DELETEs: it uses an index on column_1 of TABLE_A.
My doubt is that the query planner is not enough fast to follow sudden changes in the way I use the DB, is there a way in which I can help it to adjust its statistics and its query planner more quickly?
My other doubt is that the foreign key on TABLE_B is a problem when I try to delete from TABLE_A, and postgres tries to find nonexistent constrained rows on TABLE_B.
Thank you for our help
Gianluca Galantucci
-----Messaggio originale-----
Da: Gregory Stark [mailto:stark@enterprisedb.com]
Inviato: lunedì 17 settembre 2007 12.22
A: Heikki Linnakangas
Cc: Galantucci Giovanni; pgsql-performance@postgresql.org
Oggetto: Re: DELETE queries slow down
"Heikki Linnakangas" <heikki@enterprisedb.com> writes:
> Galantucci Giovanni wrote:
>
>> For 1 or 2 hours we update only one table, and everything goes ok, where
>> DELETE last at most 6 or 7 seconds.
>>
>> Then for a minute we do INSERT on both table, and everything continue
>> going ok, with DELETE that last about 10 seconds.
>>
>> From that moment on, DELETES become timeless, and last for 240 and more
>> seconds!
What do the inserts and deletes actually look like? Are there subqueries or
joins or are they just inserting values and deleting simple where clauses?
And are these in autocommit mode or are you running multiple commands in a
single transaction?
Generally it's faster to run more commands in a single transaction but what
I'm worried about is that you may have a transaction open which you aren't
committing for a long time. This can stop vacuum from being able to clean up
dead space and if it's in the middle of a query can actually cause vacuum to
get stuck waiting for the query to finish using the page it's using.
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Internet Email Confidentiality Footer
-----------------------------------------------------------------------------------------------------
La presente comunicazione, con le informazioni in essa contenute e ogni documento o file allegato, e' rivolta unicamente alla/e persona/e cui e' indirizzata ed alle altre da questa autorizzata/e a riceverla. Se non siete i destinatari/autorizzati siete avvisati che qualsiasi azione, copia, comunicazione, divulgazione o simili basate sul contenuto di tali informazioni e' vietata e potrebbe essere contro la legge (art. 616 C.P., D.Lgs n. 196/2003 Codice in materia di protezione dei dati personali). Se avete ricevuto questa comunicazione per errore, vi preghiamo di darne immediata notizia al mittente e di distruggere il messaggio originale e ogni file allegato senza farne copia alcuna o riprodurne in alcun modo il contenuto.
This e-mail and its attachments are intended for the addressee(s) only and are confidential and/or may contain legally privileged information. If you have received this message by mistake or are not one of the addressees above, you may take no action based on it, and you may not copy or show it to anyone; please reply to this e-mail and point out the error which has occurred.
-----------------------------------------------------------------------------------------------------
^ permalink raw reply [nested|flat] 2+ messages in thread
* Re: R: DELETE queries slow down
2007-09-18 07:17 R: DELETE queries slow down Galantucci Giovanni <Giovanni.Galantucci@italtel.it>
@ 2007-09-18 16:18 ` Bill Moran <wmoran@collaborativefusion.com>
0 siblings, 0 replies; 2+ messages in thread
From: Bill Moran @ 2007-09-18 16:18 UTC (permalink / raw)
To: Galantucci Giovanni <Giovanni.Galantucci@italtel.it>; +Cc: pgsql-performance
In response to "Galantucci Giovanni" <Giovanni.Galantucci@italtel.it>:
> I perform simple INSERT and simple where-clause DELETE.
> I also force a commit after every DELETE.
Do you mean that you delete 1 row at a time? This is slower than
batching your deletes.
> My two tables are about these:
>
> TABLE_A
> Column_1 | column2 | .......
>
> TABLE_B
> Column_1B foreign key references TABLE_A(column_1) on delete cascade | .........
>
> Every row in TABLE_B is also present in TABLE_A, but the contrary is not true.
> After hours in which I insert and delete only on TABLE_A (everything ok), I start inserting also on TABLE_B, exploiting the constrain on column_1B. After the first DELETE I perform on both tables, each following DELETE lasts for minutes, with cpu usage on 99,9%.
> I tried also to perform a VACUUM after each DELETE, but had no benefits.
> Even the EXPLAIN ANALYZE of the DELETE shows no changes with respect to the previous DELETEs: it uses an index on column_1 of TABLE_A.
Are you unable to provide these details? (i.e. output of explain, the
actual table schema, actual queries) Without them, the question is
very vague and difficult to give advice on.
If the planner comes up with the same plan whether running fast or slow,
the question is what part of that plan is no longer valid (what part's
actual time no longer matches it's predicted time)
> My doubt is that the query planner is not enough fast to follow sudden changes in the way I use the DB, is there a way in which I can help it to adjust its statistics and its query planner more quickly?
See:
http://www.postgresql.org/docs/8.2/static/sql-analyze.html
which also has links to other information on this topic.
If you can demonstrate that the statistics are stale, you might benefit
from manual analyze after large operations.
> My other doubt is that the foreign key on TABLE_B is a problem when I try to delete from TABLE_A, and postgres tries to find nonexistent constrained rows on TABLE_B.
It's quite possible, considering the fact that you seem to be CPU bound.
>
> -----Messaggio originale-----
> Da: Gregory Stark [mailto:stark@enterprisedb.com]
> Inviato: lunedì 17 settembre 2007 12.22
> A: Heikki Linnakangas
> Cc: Galantucci Giovanni; pgsql-performance@postgresql.org
> Oggetto: Re: DELETE queries slow down
>
> "Heikki Linnakangas" <heikki@enterprisedb.com> writes:
>
> > Galantucci Giovanni wrote:
> >
> >> For 1 or 2 hours we update only one table, and everything goes ok, where
> >> DELETE last at most 6 or 7 seconds.
> >>
> >> Then for a minute we do INSERT on both table, and everything continue
> >> going ok, with DELETE that last about 10 seconds.
> >>
> >> From that moment on, DELETES become timeless, and last for 240 and more
> >> seconds!
>
> What do the inserts and deletes actually look like? Are there subqueries or
> joins or are they just inserting values and deleting simple where clauses?
>
> And are these in autocommit mode or are you running multiple commands in a
> single transaction?
>
> Generally it's faster to run more commands in a single transaction but what
> I'm worried about is that you may have a transaction open which you aren't
> committing for a long time. This can stop vacuum from being able to clean up
> dead space and if it's in the middle of a query can actually cause vacuum to
> get stuck waiting for the query to finish using the page it's using.
--
Bill Moran
Collaborative Fusion Inc.
http://people.collaborativefusion.com/~wmoran/
wmoran@collaborativefusion.com
Phone: 412-422-3463x4023
^ permalink raw reply [nested|flat] 2+ messages in thread
end of thread, other threads:[~2007-09-18 16:18 UTC | newest]
Thread overview: 2+ messages (download: mbox mbox.gz follow: Atom feed)
-- links below jump to the message on this page --
2007-09-18 07:17 R: DELETE queries slow down Galantucci Giovanni <Giovanni.Galantucci@italtel.it>
2007-09-18 16:18 ` Bill Moran <wmoran@collaborativefusion.com>
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox