Re: Detach/attach database

On Mon, Nov 14, 2011 at 4:55 AM, Thom Brown <thom(at)linux(dot)com> wrote:
> So can I humbly request we completely re-architect the whole of
> PostgreSQL to fit this feature?  Thanks.

Heh.

I have to admit I've thought about this from time to time, and it
would be pretty cool. I was initially thinking that it wouldn't be
that difficult to do this on a per-database level, because if you
slurp up a whole database then by definition you're also including the
system catalogs, which means that you have the pg_class, pg_attribute,
and pg_type entries that are necessary to interpret the table
contents. If you do anything more fine-grained (per-tablespace,
per-table, or whatever) then things get much more complex, but at the
database level you only need to worry about interactions with other
globals: tablespace and role definitions. And we could probably write
code to grovel through the system catalogs for a newly "mounted"
database and do search and replace on the appropriate columns, to map
from the old OIDs to the new ones. It wouldn't be simple, but I think
it could be done.

But Tom's point about XIDs and LSNs seems like it kind of puts a
bullet through the heart of the whole idea. Now, before you can move
the database (or table, or whatever) between clusters, you've got to
rewrite all the data files to freeze XIDs and, I don't know, zero out
LSNs, or something. And if you're going to rewrite all the data, then
you've pretty much lost all the benefit of doing this in the first
place. In fact, it might end up being *slower* than a dump and
restore; even an uncompressed dump will be smaller than the on-disk
footprint of the original database, and many dumps compress quite
well.

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