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Re: [GENERAL] pgstattuple triggered checkpoint failure and database outage?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Stuart Bishop <stuart(at)stuartbishop(dot)net>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: [GENERAL] pgstattuple triggered checkpoint failure and database outage?
Date: 2009-03-31 13:57:46
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Lists: pgsql-generalpgsql-hackers
Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
> Tom Lane wrote:
>> A quick look at contrib/pgstattuple shows that it makes no effort
>> whatsoever to avoid reading temp tables belonging to other sessions.

> contrib/pageinspect has the same bug. Not surprising as it was largely 
> inspired by pgstattuple.

Given the seriousness of the consequences (forced database shutdown is
no fun), I wonder whether we should install some low-level defense
against this type of problem; ie teach ReadBuffer to throw error if
asked to read a block from someone else's temp table.

This isn't entirely trivial because it's presently expensive to
determine whether a table is someone else's temp table: it takes a
system catalog lookup.  I'm not even sure that it'd be safe to have
the relcache do it and cache the result --- it could lead to infinite
recursion.  (At the very least this would promote pg_namespace into
the set of critical relcache entries.)

The solution that seems most practical to me is to add a bool column
to pg_class indicating "this is a temp table".  Then, if that flag
is set but it's not our own temp table (which we can tell easily),
refuse to read.  However, a patch of that size would take a little
while to develop, and I'm not entirely sure it's worth the trouble.
I can't remember having seen bugs of this type before.


			regards, tom lane

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