Re: DELETE taking too much memory

On Friday 08 July 2011 10:05:47 Dean Rasheed wrote:
> > On Thu, 2011-07-07 at 15:34 +0200, vincent dephily wrote:
> >> Hi,
> >>
> >> I have a delete query taking 7.2G of ram (and counting) but I do not
> >> understant why so much memory is necessary. The server has 12G, and
> >> I'm afraid it'll go into swap. Using postgres 8.3.14.
> >>
> >> I'm purging some old data from table t1, which should cascade-delete
> >> referencing rows in t2. Here's an anonymized rundown :
> >>
> >> # explain delete from t1 where t1id in (select t1id from t2 where
> >> foo=0 and bar < '20101101');
>
> It looks as though you're hitting one of the known issues with
> PostgreSQL and FKs. The FK constraint checks and CASCADE actions are
> implemented using AFTER triggers, which are queued up during the query
> to be executed at the end. For very large queries, this queue of
> pending triggers can become very large, using up all available memory.
>
> There's a TODO item to try to fix this for a future version of
> PostgreSQL (maybe I'll have another go at it for 9.2), but at the
> moment all versions of PostgreSQL suffer from this problem.

That's very interesting, and a more plausible not-optimized-yet item than my
guesses so far, thanks. Drop me a mail if you work on this, and I'll find some
time to test your code.

I'm wondering though : this sounds like the behaviour of a "deferrable" fkey,
which AFAICS is not the default and not my case ? I haven't explored that area
of constraints yet, so there's certainly some detail that I'm missing.

> The simplest work-around for you might be to break your deletes up
> into smaller chunks, say 100k or 1M rows at a time, eg:
>
> delete from t1 where t1id in (select t1id from t2 where foo=0 and bar
> < '20101101' limit 100000);

Yes, that's what we ended up doing. We canceled the query after 24h, shortly
before the OOM killer would have, and started doing things in smaller batches.

--
Vincent de Phily