Re: Info about concurrent sequential scans

Daniele Varrazzo <daniele(dot)varrazzo(at)gmail(dot)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).
>
> So I'd like to know:
>
> is this feature currently implemented (I'm specifically interested in PG 8.3)?

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

> Is there any prerequisite needed to benefit from it (config setting,
> query characteristic, etc.)?

No.

Andreas
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