Re: R: R: DELETE queries slow down - Mailing list pgsql-performance

"Galantucci Giovanni" <Giovanni.Galantucci@italtel.it> wrote:
>
> No, I perform a single DELETE for about 80000/100000 rows at a time.
>
> Yesterday I tried to raise the parameter default_statistics_target on the file postgresql.conf, setting it to 50
(previouslyit was set to 10) and everything went ok. 
>
> It seems that postgres needs some time to adapt itself to sudden changes in the way I use the DB, maybe to adapt its
plannerto the new way of use. I think that tuning this parameter could be enough to help postgres update it's planner
faster.
>
> Do you think it could be reasonable?

Based on the information you've given and the responses you've made,
I think you're as likely to roll a 1d6 and get the right solution as
anything else.

Good luck.

> -----Messaggio originale-----
> Da: Bill Moran [mailto:wmoran@collaborativefusion.com]
> Inviato: martedì 18 settembre 2007 18.19
> A: Galantucci Giovanni
> Cc: pgsql-performance@postgresql.org
> Oggetto: Re: [PERFORM] R: DELETE queries slow down
>
>
>
> 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,
exploitingthe 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_1of 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 the question is what part of that plan is no longer valid (what part's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
wayin 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
triesto 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
>
>
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--
Bill Moran
Collaborative Fusion Inc.

wmoran@collaborativefusion.com
Phone: 412-422-3463x4023