On 12.12.2014 14:19, Robert Haas wrote: > On Thu, Dec 11, 2014 at 5:46 PM, Tomas Vondra <t...@fuzzy.cz> wrote: > >> Regarding the "sufficiently small" - considering today's hardware, we're >> probably talking about gigabytes. On machines with significant memory >> pressure (forcing the temporary files to disk), it might be much lower, >> of course. Of course, it also depends on kernel settings (e.g. >> dirty_bytes/dirty_background_bytes). > > Well, this is sort of one of the problems with work_mem. When we > switch to a tape sort, or a tape-based materialize, we're probably far > from out of memory. But trying to set work_mem to the amount of > memory we have can easily result in a memory overrun if a load spike > causes lots of people to do it all at the same time. So we have to > set work_mem conservatively, but then the costing doesn't really come > out right. We could add some more costing parameters to try to model > this, but it's not obvious how to get it right.
Ummm, I don't think that's what I proposed. What I had in mind was a flag "the batches are likely to stay in page cache". Because when it is likely, batching is probably faster (compared to increased load factor). Tomas -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
