Re: ALTER TABLE lock strength reduction patch is unsafe

On Thu, Jun 16, 2011 at 11:54 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> 2. In response, some other backend starts to reload its relcache entry
> for pgbench_accounts when it begins its next command.  It does an
> indexscan with SnapshotNow on pg_class to find the updated pg_class row.
>
> 3. Meanwhile, some third backend commits another ALTER TABLE, updating
> the pg_class row another time.  Since we have removed the
> AccessExclusiveLock that all variants of ALTER TABLE used to take, this
> commit can happen while backend #2 is in process of scanning pg_class.

This part is the core of the problem:

We must not be able to update the catalog entry while a relcache
rebuild scan is in place.

So I'm prototyping something that allows
LockRelationDefinitionOid(targetRelId, ShareLock);

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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