Re: possible wrong query plan on pg 8.3.5,

Цитат от Robert Haas <robertmhaas(at)gmail(dot)com>:

> 2009/9/14 <zz_11(at)mail(dot)bg>:
>> Цитат от Robert Haas <robertmhaas(at)gmail(dot)com>:
>>
>>> 2009/9/14  <tv(at)fuzzy(dot)cz>:
>>>>
>>>> It seems there's something very wrong - the plans are "equal" but in the
>>>> first case the results (actual time) are multiplied by 100. Eithere there
>>>> is some sort of cache (so the second execution is much faster), or the
>>>> system was busy during the first execution, or there is something wrong
>>>> with the hardware.
>>>
>>> I think you should run this query more than twice.  If it's slow the
>>> first time and fast every time for many executions after that, then
>>> it's probably just the data getting loaded into the OS cache (or
>>> shared buffers).  If it's bouncing back and forth between fast and
>>> slow, you might want to check whether your machine is swapping.
>>
>> I did it many times. Alter the first atempt it works fast, but after a
>> couple of minutes ( I think after changing the data in cache) the query is
>> working also very slow.
>>
>> I do not see any swap on OS.
>>
>>>
>>> It might also be helpful to post all the uncommented settings from
>>> your postgresql.conf file.
>>
>> postgresql.conf :
>>
>> max_connections = 2000
>> shared_buffers = 1800MB
>> temp_buffers = 80MB
>> work_mem = 120MB
>>
>> maintenance_work_mem = 100MB
>> max_fsm_pages = 404800
>> max_fsm_relations = 5000
>>
>> max_files_per_process = 2000
>> wal_buffers = 64MB
>> checkpoint_segments = 30
>> effective_cache_size = 5000MB
>> default_statistics_target = 800
>
> I think you're exhausting the physical memory on your machine. How
> much RAM do you have? How many active connections at one time? 120MB
> is a HUGE value for work_mem. I would try reducing that to, say, 4
> MB, and see what happens. Your setting for temp_buffers also seems
> way too high. I would put that one back to the default, at least for
> starters. And for that matter, why have you increased the value for
> wal_buffers to over 1000 times the default value?
>

We have 8 GB RAM, running Centos 64-bit and ~10 to 15 active
connections ( using connection pool).
120 MB for work mem is good. If I drop this value I will receive very
bad performance for the hole system.

I will try to reduce wal_buffers ( is this value connected to ram usage ? ).

> The reason you may not be seeing evidence of swapping is that it may
> be happening quite briefly during query execution. But I have to
> think it's happening, because otherwise the performance drop-off is
> hard to account for.
>
On linux if I have swap the os never restores the ram used for swap.
And I do not see any swap on OS. I send the vmstat for the server:
0 0 1388 44852 25160 6225316 0 0 304 0 1018 201 0
0 100 0 0
procs -----------memory---------- ---swap-- -----io---- --system--
-----cpu------
r b swpd free buff cache si so bi bo in cs us
sy id wa st
0 0 1388 47612 25148 6222364 0 0 332 4 1015 194 0
0 100 0 0
0 0 1388 47072 25156 6222900 0 0 268 8 1015 190 0
0 100 0 0
0 0 1388 46532 25160 6223656 0 0 270 0 1014 194 0
0 100 0 0

> ...Robert
>
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