Re: Info about concurrent sequential scans

On Mon, Feb 22, 2010 at 7:59 PM, Andreas Kretschmer
<akretschmer@spamfence.net> wrote:
> Daniele Varrazzo <daniele.varrazzo@gmail.com> wrote:
>
>> Hello,
>>
>> at Prato PgDay in 2007 I remember hearing in a speech about a (then
>> yet to come) "seqscan piggyback" feature, allowing concurrent
>> sequential scans to use the same disk reads. I've now googled for info
>> about this feature, but I found nothing conclusive (e.g. [1], [2] -
>> which I don't know where is linked).

> I think, you means this:
>
>  Concurrent large sequential scans can now share disk reads (Jeff Davis)
>
> This is accomplished by starting the new sequential scan in the middle
> of the table (where another sequential scan is already in-progress) and
> wrapping around to the beginning to finish. This can affect the order of
> returned rows in a query that does not specify ORDER BY. The
> synchronize_seqscans configuration parameter can be used to disable this
> if necessary
>
> Source:
> http://www.postgresql.org/docs/current/interactive/release-8-3.html

Thank you very much (and thanks to Bruce too). This is the feature and
I'm checking it works "out of the box" on PG 8.3

-- Daniele