Hi Joachim,

On Wed, 10 Feb 2010 11:36:41 +0100, Joachim Wieland <j...@mcknight.de>
http://www.postgresql.org/docs/8.4/static/backup-dump.html already
states about pg_dump: "In particular, it must have read access to all
tables that you want to back up, so in practice you almost always have
to run it as a database superuser." so I think there is not a big loss

Hm.. I doubt somewhat that's common practice. After all, read access to all tables is still a *lot* less than superuser privileges. But yeah, the documentation currently states that.

They more or less get it "by chance" :-)  They acquire a snapshot when
they call pg_synchronize_snapshot_taken()

Oh, I see, calling the function by itself already acquires a snapshot.
Even in case of a fast path call, it seems. Then your approach is correct.

(I'd still feel more comfortable, it I had seen a GetTransactionSnapshot() or something akin in there).

and if all the backends do
it while the other backend holds the lock in shared mode, we know that
the snapshot won't change, so they all get the same snapshot.

Agreed, that works.

(Ab)using the ProcArrayLock for synchronization is probably acceptable for pg_dump, however, I'd rather take another approach for a more general implementation.

Also, you should probably ensure the calling transactions don't have a
snapshot already (let alone a transaction id).


Hm.. realizing that a function call per-se acquires a snapshot, I fail to see how we could check if we really acquired a snapshot. Consider the following (admittedly stupid) example:

SELECT version();
 ... time goes by ...
SELECT pg_synchronize_snapshot_taken(..);

As it stands, your function would silently fail to "synchronize" the
snapshots, if other transactions committed in between the two function calls.

It seemed more robust and convenient to have an expiration in the
backend itself. What would happen if you called
pg_synchronize_snapshots() and if right after that your network
connection dropped? Without the server noticing, it would continue to
hold the lock and you could not log in anymore...

Hm.. that's a point. Given this approach uses the ProcArrayLock, it's
probably better to use an explicit timeout.

But you are right: The proposed feature is a pragmatic and quick
solution for pg_dump and similar but we might want to have a more
general snapshot cloning procedure instead. Not having a delay for
other activities at all and not requiring superuser privileges would
be a big advantage over what I have proposed.



Markus Wanner

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