Re: Same SQL, 104296ms of difference between 7.4.12 and

Rafael Martinez Guerrero wrote:
> Hello
>
> I have a sql statement that takes 108489.780 ms with 8.0.7 in a
> RHEL4/amd64linux server with 2xAMD Opteron(tm) Processor 275 2.00GHz /
> 8GB RAM and only 4193.588 ms with 7.4.12 in a RHEL3/386linux server with
> 2xIntel(R) Xeon(TM) CPU 2.40GHz / 4GB RAM.
>
> Some information:
>
> - There is no IO when I am running the sql, but it uses 99% of the cpu.
> - I run VACUUM VERBOSE ANALYZE in both databases before the test.
> - The databases are identical.
> - No other jobs running when testing.
> - Some different parameters between 7.4.12 and 8.0.7 :
>
> 7.4.12:
> -------
> shared_buffers = 114966 #(15% of ram)
> sort_mem = 16384
> vacuum_mem = 524288
> wal_buffers = 64
> checkpoint_segments = 16
> effective_cache_size = 383220 #(50% ram)
> random_page_cost = 3
> default_statistics_target = 100
>
> 8.0.7:
> ------
> shared_buffers = 250160 #(25% ram)
> work_mem = 8192
> maintenance_work_mem = 131072
> wal_buffers = 128
> checkpoint_segments = 64
> effective_cache_size = 500321 #(50% ram)
> random_page_cost = 3
> default_statistics_target = 100
>
> Any ideas of what I can test/configurate to find out why this happens?
> Thanks in advance.

I haven't looked in detail at the plans, but what stands out to me is
that you've got a sort with a lot of columns and you've halved sort_mem
(work_mem). Try increasing it (perhaps to 32000 even).
set work_mem = 32000;

Give that a quick go and see what happens. If it doesn't work, we'll
look at the plans in more detail.
--
Richard Huxton
Archonet Ltd