On Thu, Jun 16, 2011 at 11:54 PM, Tom Lane <tgl@sss.pgh.pa.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/
PostgreSQL Development, 24x7 Support, Training & Services
