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Re: Dynamic Partitioning using Segment Visibility Maps

From: Markus Schiltknecht <markus(at)bluegap(dot)ch>
To: Gokulakannan Somasundaram <gokul007(at)gmail(dot)com>
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 10:39:47
Message-ID: 4780AFF3.7070904@bluegap.ch (view raw or flat)
Thread:
Lists: pgsql-hackers
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.

Regards

Markus


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