On 12.12.2014 14:19, Robert Haas wrote:
> On Thu, Dec 11, 2014 at 5:46 PM, Tomas Vondra <tv@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