On Sat, Apr 20, 2019 at 10:36:50PM +0200, Tomas Vondra wrote:
>On Sat, Apr 20, 2019 at 04:26:34PM -0400, Tom Lane wrote:
>>Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
>>>Considering how rare this issue likely is, we need to be looking for a
>>>solution that does not break the common case.
>>
>>Agreed. What I think we need to focus on next is why the code keeps
>>increasing the number of batches. It seems like there must be an undue
>>amount of data all falling into the same bucket ... but if it were simply
>>a matter of a lot of duplicate hash keys, the growEnabled shutoff
>>heuristic ought to trigger.
>>
>
>I think it's really a matter of underestimate, which convinces the planner
>to hash the larger table. In this case, the table is 42GB, so it's
>possible it actually works as expected. With work_mem = 4MB I've seen 32k
>batches, and that's not that far off, I'd day. Maybe there are more common
>values, but it does not seem like a very contrived data set.
>
Actually, I might have spoken too soon. I've dne some stats on the sample
data. There are 113478127 rows in total, and while most UUIDs are unique,
there are UUIDs that represent ~10% of the data. So maybe there really is
something broken in disabling the growth.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
