Re: Hash join gets slower as work_mem increases?

Tomas Vondra wrote:
> On 01/29/2016 04:17 PM, Albe Laurenz wrote:
>> I have a query that runs *slower* if I increase work_mem.
>>
>> The execution plans are identical in both cases, except that a temp file
>> is used when work_mem is smaller.

>> What could be an explanation for this?
>> Is this known behaviour?
>
> There is a bunch of possible causes for such behavior, but it's quite
> impossible to say if this is an example of one of them as you have not
> posted the interesting parts of the explain plan. Also, knowing
> PostgreSQL version would be useful.
>
> I don't think the example you posted is due to exceeding on-CPU cache as
> that's just a few MBs per socket, so the smaller work_mem is
> significantly larger.
>
> What I'd expect to be the issue here is under-estimate of the hash table
> size, resulting in too few buckets and thus long chains of tuples that
> need to be searched sequentially. Smaller work_mem values usually limit
> the length of those chains in favor of batching.
>
> Please, post the whole explain plan - especially the info about number
> of buckets/batches and the Hash node details.

Thanks for looking at this.
Sorry, I forgot to mention that this is PostgreSQL 9.3.10.

I didn't post the whole plan since it is awfully long, I'll include hyperlinks
for the whole plan.

work_mem = '100MB' (http://explain.depesz.com/s/7b6a):

-> Hash Join (cost=46738.74..285400.61 rows=292 width=8) (actual time=4296.986..106087.683 rows=187222 loops=1)
Hash Cond: ("*SELECT* 1_2".postadresse_id = p.postadresse_id)
Buffers: shared hit=1181177 dirtied=1, temp read=7232 written=7230
[...]
-> Hash (cost=18044.92..18044.92 rows=4014 width=8) (actual time=4206.892..4206.892 rows=3096362 loops=1)
Buckets: 1024 Batches: 2 (originally 1) Memory Usage: 102401kB
Buffers: shared hit=1134522 dirtied=1, temp written=5296

work_mem = '500MB' (http://explain.depesz.com/s/Cgkl):

-> Hash Join (cost=46738.74..285400.61 rows=292 width=8) (actual time=3802.849..245970.049 rows=187222 loops=1)
Hash Cond: ("*SELECT* 1_2".postadresse_id = p.postadresse_id)
Buffers: shared hit=1181175 dirtied=111
[...]
-> Hash (cost=18044.92..18044.92 rows=4014 width=8) (actual time=3709.584..3709.584 rows=3096360 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 120952kB
Buffers: shared hit=1134520 dirtied=111

Does that support your theory?

There is clearly an underestimate here, caused by correlated attributes, but
is that the cause for the bad performance with increased work_mem?

Yours,
Laurenz Albe