Re: Catalog/Metadata consistency during changeset extraction from wal

On Sunday, June 24, 2012 11:37:26 PM Simon Riggs wrote:
> On 24 June 2012 22:11, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> > One interesting problem are table rewrites (truncate, cluster, some ALTER
> > TABLE's) and dropping tables. Because we nudge SnapshotNow to the past
> > view it had back when the wal record was created we get the old
> > relfilenode. Which might have been dropped in part of the transaction
> > cleanup...
> > With most types thats not a problem. Even things like records and arrays
> > aren't problematic. More interesting cases include VACUUM FULL $systable
> > (e.g. pg_enum) and vacuum full'ing a table which is used in the *_out
> > function of a type (like a user level pg_enum implementation).

> That's only a problem if you are generating changes to the relfilenode
> rather than the relid.
Hm. I can't follow so far. Could you paraphrase?

> ISTM that this step differs depending upon whether we are generating
> portable SQL, or whether we are generating changes for immediate
> apply.
I fear only generating changes for immediate, low-level apply is going to fly
given the various interests people have voiced.

> If it is the latter, then it should never actually happen because if a table
> rewrite occurred and then committed we would never need to re-read earlier
> WAL.
> So treating this as a generic problem leads to some weird results that
> we don't need to worry about cos they can't actually happen.
Well, even if it were true that we don't need to worry about the state before
a full-table rewritte - I don't think it is - we still need to be able to cope
with CLUSTER or VACUUM FULL...

Greetings,

Andres
--
Andres Freund http://www.2ndQuadrant.com/
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