On Fri, Jul 8, 2011 at 4:35 AM, Dean Rasheed <dean.a.rash...@gmail.com>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. > > 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); > I'd like to comment here.... I had serious performance issues with a similar query (planner did horrible things), not sure if planner will do the same dumb thing it did for me, my query was against the same table (ie, t1=t2). I had this query: delete from t1 where ctid in (select ctid from t1 where created_at<'20101231' limit 10000); <--- this was slooooow. Changed to: delete from t1 where ctid = any(array(select ctid from t1 where created_at<'20101231' limit 10000)); <--- a lot faster. So... will the same principle work here?, doing this?: delete from t1 where t1id = any(array(select t1id from t2 where foo=0 and bar < '20101101' limit 100000)); <-- would this query be faster then original one? > > Regards, > Dean > > -- > Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-performance >
