Re: query slowdown after 9.0 -> 9.4 migration

On 10/26/2016 03:48 PM, Filip Rembiałkowski wrote:
> Hi.
>
> Query run time degraded after migration from Pg 9.0 + postgis 1.5 to Pg
> 9.4 + postgis 2.2.
>
> 1 ms versus 7 ms.
>
> Same query, same data, same schema, similar hardware. Data is small and
> fits in cache.
>
> EXPLAIN shows heap scan cost increase. What can be the reason for
> 40-fold increase in page scans needed to run Bitmap Heap Scan with
> Filter and Recheck?
>

On 9.0 the the scan accesses only 8 buffers:

Buffers: shared hit=8

while on 9.4 it has to inspect 316 of them:

Buffers: shared hit=316

Perhaps the table is organized / sorted differently, or something like
that. How did you do the upgrade?


ragards

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