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Re: Dynamic Partitioning using Segment Visibility Maps

From: "Gokulakannan Somasundaram" <gokul007(at)gmail(dot)com>
To: "Markus Schiltknecht" <markus(at)bluegap(dot)ch>
Cc: tomas(at)tuxteam(dot)de, "Simon Riggs" <simon(at)2ndquadrant(dot)com>, "Robert Treat" <xzilla(at)users(dot)sourceforge(dot)net>, pgsql-hackers(at)postgresql(dot)org, "Andrew Sullivan" <ajs(at)crankycanuck(dot)ca>
Subject: Re: Dynamic Partitioning using Segment Visibility Maps
Date: 2008-01-06 11:13:47
Message-ID: 9362e74e0801060313yfb55391y60cb271eb37a34fd@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Jan 6, 2008 4:09 PM, Markus Schiltknecht <markus(at)bluegap(dot)ch> wrote:

> Hi,
>
> Gokulakannan Somasundaram wrote:
> >
> >
> > On Jan 5, 2008 6:15 PM, <tomas(at)tuxteam(dot)de <mailto:tomas(at)tuxteam(dot)de>>
> wrote:
> >
> >
> >     One thought I had back then, with partitioned tables was "gee --
> B-tree
> >     index is already doing a partition; why do a manual partition on top
> of
> >     that?".
> >
> > Can you please explain more on what you are trying to say here?
>
> I think this has to do with SE not being of much use for index scans. Or
> put it another way: SE is an optimization for sequential scans. For
> tables where it works well, it could possibly replace the index entirely.
>
> Without the index, you would rely on SE to always be able to exclude
> enough segments, so that the seq scan is less expensive than an index
> scan with the following table lookups.
>
> With an index, the planner gets a hard time deciding  between the index
> scan and the (possibly SE optimized) seq scan.


That's a good point. But i think Simon is planning not to give the job to
the planner, but to the executor. So SE optimization will come into play,
only when planner has decided on Sequential scan.

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