From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Martijn van Oosterhout <kleptog(at)svana(dot)org> |
Cc: | "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>, Wes <wespvp(at)syntegra(dot)com>, Zeugswetter Andreas DCP SD <ZeugswetterA(at)spardat(dot)at>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Catalog Access (was: [GENERAL] Concurrency problem building indexes) |
Date: | 2006-04-25 17:58:27 |
Message-ID: | 22715.1145987907@sss.pgh.pa.us |
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Martijn van Oosterhout <kleptog(at)svana(dot)org> writes:
> I think the basic problem is that DDL can't really work within a
> transaction. If I do an ALTER TABLE, some of these changes need to show
> up to concurrent transactions (maybe creating a unique index?).
The point is that DDL can't be MVCC. If for instance you add an index
to a table, once you commit every other transaction must *immediately*
start updating that index when they modify the table. They can't use
the excuse of "not my problem because the catalog change postdates the
snapshot I'm using". The drop-index case is even worse, since a
transaction that believes the index is still present is likely to try
to access/update a disk file that's not there anymore. Adding/dropping
columns, constraints, triggers, etc all have hazards of the same ilk.
> I think it's like Tom says in that email, it could be done, but the
> cost/benefit ratio isn't very good...
It's barely possible that we could make this happen, but it would be a
huge amount of work, and probably a huge amount of instability for a
very long time until we'd gotten all the corner cases sorted. I think
there are much more productive uses for our development effort.
regards, tom lane
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