On 02/27/2017 12:55 PM, Andres Freund wrote:
On 2017-02-24 15:18:04 -0800, Andres Freund wrote:
On 2017-02-24 15:12:37 -0800, Andres Freund wrote:
On 2017-02-24 18:04:18 -0500, Tom Lane wrote:
Concretely, something like the attached.  This passes regression tests
but I've not pushed on it any harder than that.

Heh, I'd just gotten something that didn't immediately crash anymore ;)

Running your patch against Jeff's test-case, verified before that I
could easily reproduce the O(N^2) cost.

Oh, that didn't take as long as I was afraid (optimized/non-assert build):

postgres[26268][1]=# SET work_mem = '13GB';
Time: 2.591 ms
postgres[26268][1]=# select count(*) from foobar2 where not exists (select 1 
from foobar t where t.titleid=foobar2.titleid);
Time: 268043.710 ms (04:28.044)

As another datapoint, I measured this patch against the problem from
(see top post in thread), and it indeed fixes the runtime issue -
there's still considerably higher memory usage and some runtime
overhead, but the quadratic behaviour is gone.

I think we should go forward with something like this patch in all
branches, and only use Tomas' patch in master, because they're
considerably larger.

So you've tried to switch hashjoin to the slab allocators? Or what have you compared?


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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