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From: Gregory Stark <stark@enterprisedb.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Steven Flatt <steven.flatt@gmail.com>
Cc: Mark Kirkwood <markir@paradise.net.nz>
Cc: Bill Moran <wmoran@collaborativefusion.com>
Cc: Vivek Khera <vivek@khera.org>
Cc: Pgsql performance <pgsql-performance@postgresql.org>
Subject: Re: When/if to Reindex
Date: Fri, 24 Aug 2007 21:06:53 +0100
Message-ID: <876434zogy.fsf@oxford.xeocode.com> (raw)
In-Reply-To: <2774.1187976535@sss.pgh.pa.us>
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	<357fa7590708090651p71b9bc47n66739f8dbbb4ff57@mail.gmail.com>
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	<17370.1187927942@sss.pgh.pa.us>
	<46CE6883.10104@paradise.net.nz>
	<357fa7590708240722m204d678eid02b7ce3470eba1a@mail.gmail.com>
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	<357fa7590708240956k18e3ba7dv735cdaa99a667312@mail.gmail.com>
	<2774.1187976535@sss.pgh.pa.us>


"Tom Lane" <tgl@sss.pgh.pa.us> writes:

> "Steven Flatt" <steven.flatt@gmail.com> writes:
>> So, can we simply trust what's in pg_class.relpages and ignore looking
>> directly at the index?
>
> No, we can't.  In the light of morning I remember more about the reason
> for the aforesaid patch: it's actually unsafe to read the pg_class row
> at all if you have not got lock on the index.  We are reading with
> SnapshotNow in order to be sure we see up-to-date info, and that means
> that a concurrent update of the row (eg, for REINDEX to report the new
> relfilenode) can have the following behavior:

Should reindex be doing an in-place update? Don't we have to do in-place
updates for other system catalogs which are read in snapshotnow for precisely
the same reasons?

Alternatively, why does the planner need access to the pg_class entry and not
just the pg_index record?

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com



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