Re: Better support of exported snapshots with pg_dump

On Mon, Sep 1, 2014 at 5:30 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> Hi,
>
>> >Currently pg_dump does not allow a user to specify an exported snapshot
>> >name that he would like to use for a dump using SET TRANSACTION SNAPSHOT
>> >(now pg_export_snapshot is only used for parallel pg_dump within it). I
>> >imagine that this would be handy to take a consistent dump of a given
>> >database after creating a logical replication slot on it. Thoughts?
>
> Yes, I always wanted that option.

I didn't find that option to be terribly important then, but I don't
see how we can possibly get by without it now, unless our goal is to
make logical decoding as hard to use as we possibly can.

Tom's got a good point about the order of locking vs. snapshot taking,
but I think the way to address that is by adding some capability to
temporarily lock out all DDL on non-temporary objects across the
entire system, rather than by trying to make pg_dump (or the walsender
creating the replication slot) lock every table. Even if we could get
that to work, it still leaves the very-much-related problem that dumps
of databases containing many tables can easily exhaust the lock table.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company