Re: WIP patch for parallel pg_dump

On Mon, Dec 6, 2010 at 7:22 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Josh Berkus <josh@agliodbs.com> writes:
> >> However, if you were doing something like parallel pg_dump you could
> >> just run the parent and child instances all against the slave, so the
> >> pg_dump scenario doesn't seem to offer much of a supporting use-case for
> >> worrying about this.  When would you really need to be able to do it?
>
> > If you had several standbys, you could distribute the work of the
> > pg_dump among them.  This would be a huge speedup for a large database,
> > potentially, thanks to parallelization of I/O and network.  Imagine
> > doing a pg_dump of a 300GB database in 10min.
>
> That does sound kind of attractive.  But to do that I think we'd have to
> go with the pass-the-snapshot-through-the-client approach.  Shipping
> internal snapshot files through the WAL stream doesn't seem attractive
> to me.
>
> While I see Robert's point about preferring not to expose the snapshot
> contents to clients, I don't think it outweighs all other considerations
> here; and every other one is pointing to doing it the other way.
>
>
How about the publishing transaction puts the snapshot in a (new) system
table and passes a UUID to its children, and the joining transactions looks
for that UUID in the system table using dirty snapshot (SnapshotAny) using a
security-definer function owned by superuser.

No shared memory used, and if WAL-logged, the snapshot would get to the
slaves too.

I realize SnapshotAny wouldn't be sufficient since we want the tuple to
become invisible when the publishing transaction ends (commit/rollback),
hence something akin to (new) HeapTupleSatisfiesStillRunning() would be
needed.

Regards,
-- 
gurjeet.singh
@ EnterpriseDB - The Enterprise Postgres Company
http://www.EnterpriseDB.com

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