On 8 July 2011 10:44, Vincent de Phily <vincent.dephily@xxxxxxxxxxxxxxxxx> wrote: > 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. > Yes, it's the same issue that affects deferrable PK and FK constraints, but even non-deferrable FKs use AFTER ROW triggers that suffer from this problem. These triggers don't show up in a "\d" from psql, but they are there (try select * from pg_trigger where tgconstrrelid = 't1'::regclass) and because they fire AFTER rather than BEFORE, queuing up large numbers of them is a problem. Regards, Dean > >> 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 > -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
