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Re: Query much faster with enable_seqscan=0

From: Scott Carey <scott(at)richrelevance(dot)com>
To: Ogden <lists(at)darkstatic(dot)com>
Cc: "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>,Greg Smith <greg(at)2ndquadrant(dot)com>
Subject: Re: Query much faster with enable_seqscan=0
Date: 2010-09-24 17:34:41
Message-ID: F99BA3A3-9D60-4943-81C8-9F11CD55E1C1@richrelevance.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Sep 22, 2010, at 6:36 AM, Ogden wrote:

> 
> On Sep 21, 2010, at 2:34 PM, Ogden wrote:
> 
>> 
>> On Sep 21, 2010, at 2:16 PM, Greg Smith wrote:
>> 
>>> Joshua D. Drake wrote:
>>>> PostgreSQL's defaults are based on extremely small and some would say
>>>> (non production) size databases. As a matter of course I always
>>>> recommend bringing seq_page_cost and random_page_cost more in line.
>>>> 
>>> 
>>> Also, they presume that not all of your data is going to be in memory, and the query optimizer needs to be careful about what it does and doesn't pull from disk.  If that's not the case, like here where there's 8GB of RAM and a 7GB database, dramatic reductions to both seq_page_cost and random_page_cost can make sense.  Don't be afraid to think lowering below 1.0 is going too far--something more like 0.01 for sequential and 0.02 for random may actually reflect reality here.
>>> 
>> 
>> I have done just that, per your recommendations and now what took 14 seconds, only takes less than a second, so it was certainly these figures I messed around with. I have set:
>> 
>> seq_page_cost = 0.01           
>> random_page_cost = 0.02   
>> cpu_tuple_cost = 0.01
>> 
>> Everything seems to run faster now. I think this should be fine - I'll keep an eye on things over the next few days. 
>> 
>> I truly appreciate everyone's help. 
>> 
>> Ogden
>> 
> 
> 
> I spoke too soon - well I came in this morning and reran the query that was speeded up yesterday by a lot after tweaking those numbers. This morning the first time I ran it, it took 16 seconds whereas every subsequent run was a matter of 2 seconds. I assume there is OS caching going on for those results. Is this normal or could it also be the speed of my disks which is causing a lag when I first run it (it's RAID 5 across 6 disks). Is there any explanation for this or what should those settings really be? Perhaps 0.01 is too low?
> 
> Thank you
> 
> Ogden

When not cached, the plan with sequential scans will almost always be much faster.

When cached in memory, the ones using indexes are almost always faster. 

The tuning parameters are essentially telling postgres the likelihood of finding things on disk instead versus in memory.   The default parameters are essentially "not likely in memory, with a somewhat slow disk".




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