Re: GiST, caching, and consistency

From: Matthew Wakeling
Subject: Re: GiST, caching, and consistency
Date: ,
Msg-id: alpine.DEB.2.00.0908051449430.18938@aragorn.flymine.org
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In response to: Re: GiST, caching, and consistency  (Robert Haas)
List: pgsql-performance

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GiST, caching, and consistency  (Matthew Wakeling, )
 Re: GiST, caching, and consistency  (Robert Haas, )
  Re: GiST, caching, and consistency  (Greg Stark, )
   Re: GiST, caching, and consistency  (Matthew Wakeling, )
    Re: GiST, caching, and consistency  (Robert Haas, )
     Re: GiST, caching, and consistency  (Matthew Wakeling, )
    Re: GiST, caching, and consistency  (Tom Lane, )

On Wed, 5 Aug 2009, Robert Haas wrote:
> On Wed, Aug 5, 2009 at 6:20 AM, Matthew Wakeling<> wrote:
>> It is certainly doing a sequential scan. So are you saying that it will
>> start a sequential scan from a different part of the table each time, even
>> in the absence of other simultaneous sequential scans? Looks like I'm going
>> to have to remove the limit to get sensible results - I only added that to
>> make the query return in a sensible time for performance testing.
>>
>> Some trivial testing with "select * from location limit 10;" indicates that
>> it starts the sequential scan in the same place each time - but is this
>> different from the above query?
>
> Maybe it's because of this?
>
> http://www.postgresql.org/docs/8.3/static/runtime-config-compatible.html#GUC-SYNCHRONIZE-SEQSCANS

Thanks, we had already worked that one out. What I'm surprised about is
that it will start the sequential scan from a different part of the table
when there aren't any simultaneous scans, but not when I do the trivial
testing.

Having reduced the data quantity (so I can throw away the limit) makes my
tests produce much more consistent results. I label this problem as
solved. Thanks all.

Matthew

--
 $ rm core
 Segmentation Fault (core dumped)


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