On Wed, Dec 6, 2017 at 10:07 AM, Ildus Kurbangaliev
<i.kurbangaliev@postgrespro.ru> wrote:
> On Fri, 1 Dec 2017 21:47:43 +0100
> Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
>> +1 to do the rewrite, just like for other similar ALTER TABLE commands
>
> Ok. What about the following syntax:
>
> ALTER COLUMN DROP COMPRESSION - removes compression from the column
> with the rewrite and removes related compression options, so the user
> can drop compression method.
>
> ALTER COLUMN SET COMPRESSION NONE for the cases when
> the users want to just disable compression for future tuples. After
> that they can keep compressed tuples, or in the case when they have a
> large table they can decompress tuples partially using e.g. UPDATE,
> and then use ALTER COLUMN DROP COMPRESSION which will be much faster
> then.
>
> ALTER COLUMN SET COMPRESSION <cm> WITH <cmoptions> will change
> compression for new tuples but will not touch old ones. If the users
> want the recompression they can use DROP/SET COMPRESSION combination.
>
> I don't think that SET COMPRESSION with the rewrite of the whole table
> will be useful enough on any somewhat big tables and same time big
> tables is where the user needs compression the most.
>
> I understand that ALTER with the rewrite sounds logical and much easier
> to implement (and it doesn't require Oids in tuples), but it could be
> unusable.
The problem with this is that old compression methods can still be
floating around in the table even after you have done SET COMPRESSION
to something else. The table still needs to have a dependency on the
old compression method, because otherwise you might think it's safe to
drop the old one when it really is not. Furthermore, if you do a
pg_upgrade, you've got to preserve that dependency, which means it
would have to show up in a pg_dump --binary-upgrade someplace. It's
not obvious how any of that would work with this syntax.
Maybe a better idea is ALTER COLUMN SET COMPRESSION x1, x2, x3 ...
meaning that x1 is the default for new tuples but x2, x3, etc. are
still allowed if present. If you issue a command that only adds
things to the list, no table rewrite happens, but if you remove
anything, then it does.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company