On 11.12.2014 22:16, Robert Haas wrote:
> On Thu, Dec 11, 2014 at 2:51 PM, Tomas Vondra <tv@fuzzy.cz> wrote:
>> No, it's not rescanned. It's scanned only once (for the batch #0), and
>> tuples belonging to the other batches are stored in files. If the number
>> of batches needs to be increased (e.g. because of incorrect estimate of
>> the inner table), the tuples are moved later.
>
> Yeah, I think I sort of knew that, but I got confused. Thanks for clarifying.
>
>> The idea was that if we could increase the load a bit (e.g. using 2
>> tuples per bucket instead of 1), we will still use a single batch in
>> some cases (when we miss the work_mem threshold by just a bit). The
>> lookups will be slower, but we'll save the I/O.
>
> Yeah. That seems like a valid theory, but your test results so far
> seem to indicate that it's not working out like that - which I find
> quite surprising, but, I mean, it is what it is, right?
Not exactly. My tests show that as long as the outer table batches fit
into page cache, icreasing the load factor results in worse performance
than batching.
When the outer table is "sufficiently small", the batching is faster.
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).
If we could identify those cases (at least the "temp files > RAM") then
maybe we could do this. Otherwise we're going to penalize all the other
queries ...
Maybe the best solution for now is "increase the work_mem a bit"
recommendation.
regards
Tomas