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 -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
