Re: Custom compression methods

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> A basic problem here is that, as proposed, DROP COMPRESSION METHOD may
> break your database irretrievably. If there's no data compressed
> using the compression method you dropped, everything is cool -
> otherwise everything is broken and there's no way to recover. The
> only obvious alternative is to disallow DROP altogether (or make it
> not really DROP).

> Both of those alternatives sound fairly unpleasant to me, but I'm not
> exactly sure what to recommend in terms of how to make it better.
> Ideally anything we expose as an SQL command should have a DROP
> command that undoes whatever CREATE did and leaves the database in an
> intact state, but that seems hard to achieve in this case.

If the use of a compression method is tied to specific data types and/or
columns, then each of those could have a dependency on the compression
method, forcing a type or column drop if you did DROP COMPRESSION METHOD.
That would leave no reachable data using the removed compression method.
So that part doesn't seem unworkable on its face.

IIRC, the bigger concerns in the last discussion had to do with
replication, ie, can downstream servers make sense of the data.
Maybe that's not any worse than the issues you get with non-core
index AMs, but I'm not sure.

regards, tom lane