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

[Richard Huxton <dev(at)archonet(dot)com>]

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