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Re: Weird issue with planner choosing seq scan

From: Sean Leach <sleach(at)wiggum(dot)com>
To: Stephen Denne <Stephen(dot)Denne(at)datamail(dot)co(dot)nz>
Cc: "pgsql-performance" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Weird issue with planner choosing seq scan
Date: 2008-02-25 23:32:10
Message-ID: DF26F4B4-FCF2-40DC-89B7-859854AC628F@wiggum.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Feb 25, 2008, at 2:59 PM, Stephen Denne wrote:
>>
>
> Please know that I'm very new at advising PostgreSQL users how they  
> should tune their system...

I'd never have known it if you hadn't said anything

>
>
> My understanding of your vacuum verbose output was that it was  
> pointing out that max_fsm_pages was currently smaller than 281727,  
> so therefore there was no way it could contain mappings to all the  
> reusable space. However I don't think it is hinting at, nor  
> recommending a value that you should be using.
>
> If you do nothing, then this number of pages with reusable space  
> will probably continue to grow, therefore, it probably has been  
> growing.
>
> So, for example, if your max_fsm_pages is currently only 20000, then  
> perhaps 20000 of the 281727 pages with reusable space are in the  
> free space map. The remaining 260000 pages _may_ have been generated  
> through 20 different processes each of which created 13000 more  
> pages with reusable space than the map could reference. If that was  
> the case, then a max_fsm_pages of 33000 might be large enough.
>
> Do you see what I'm getting at?
> I think that you should do a vacuum full of that table once, then  
> monitor the number of pages in it with reusable space for a while  
> (over a few iterations of your regular processes). That should give  
> you information about how much larger your max_fsm_pages should be  
> than it currently is.

This sounds sane to me, will do.  Thanks again!



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