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Date: Tue, 14 Aug 2012 12:29:10 -0500
From: "ktm@rice.edu" <ktm@rice.edu>
To: Jeff Janes <jeff.janes@gmail.com>
Cc: Doug Hunley <doug.hunley@gmail.com>, pgsql-performance@postgresql.org
Subject: Re: cluster on conditional index?
Message-ID: <20120814172910.GP27500@aart.rice.edu>
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On Tue, Aug 14, 2012 at 10:10:47AM -0700, Jeff Janes wrote:
> On Tue, Aug 14, 2012 at 8:27 AM, Doug Hunley <doug.hunley@gmail.com> wrote:
> > According to the docs on cluster:
> > if you tend to access some data more than others, and there is an
> > index that groups them together, you will benefit from using CLUSTER
> >
> > however, this doesn't address the situation where you have a
> > conditional index.
> 
> It seems like it is not allowed.
> 
> jjanes=# create index on pgbench_accounts (aid) where bid=33;
> jjanes=# cluster pgbench_accounts USING pgbench_accounts_aid_idx ;
> ERROR:  cannot cluster on partial index "pgbench_accounts_aid_idx"
> 
> But I don't see a fundamental reason it can't be allowed, maybe
> implementing that should be on the to-do list.
> 
> Cheers,
> 
> Jeff
> 

It probably has to do with the fact that a conditional index, does
not include every possible row in the table. Although, a "cluster" of
the matching rows and then leave the rest in place, should work. How
is that for hand-waving.  :)

Regards,
Ken