Re: Weird issue with planner choosing seq scan

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!