Re: [HACKERS] Custom compression methods - Mailing list pgsql-hackers

On Mon, Dec 11, 2017 at 12:41 PM, Alexander Korotkov
<a.korotkov@postgrespro.ru> wrote:
> Thus, in your example if user would like to further change awesome
> compression for evenbetter compression, she should write.
>
> SET COMPRESSION evenbetter PRESERVE pglz, awesome; -- full list of previous
> compression methods

Right.

> I wonder what should we do if user specifies only part of previous
> compression methods?  For instance, pglz is specified but awesome is
> missing.
>
> SET COMPRESSION evenbetter PRESERVE pglz; -- awesome is missing
>
> I think we should trigger an error in this case.  Because query is specified
> in form that is assuming to work without table rewrite, but we're unable to
> do this without table rewrite.

I think that should just rewrite the table in that case.  PRESERVE
should specify the things that are allowed to be preserved -- its mere
presence should not be read to preclude a rewrite.  And it's
completely reasonable for someone to want to do this, if they are
thinking about de-installing awesome.

> I also think that we need some way to change compression method for multiple
> columns in a single table rewrite.  Because it would be way more efficient
> than rewriting table for each of columns.  So as an alternative of
>
> ALTER TABLE tbl ALTER COLUMN c1 SET COMPRESSION awesome; -- first table
> rewrite
> ALTER TABLE tbl ALTER COLUMN c2 SET COMPRESSION awesome; -- second table
> rewrite
>
> we could also provide
>
> ALTER TABLE tbl ALTER COLUMN c1 SET COMPRESSION awesome PRESERVE pglz; -- no
> rewrite
> ALTER TABLE tbl ALTER COLUMN c2 SET COMPRESSION awesome PRESERVE pglz; -- no
> rewrite
> VACUUM FULL tbl RESET COMPRESSION PRESERVE c1, c2; -- rewrite with
> recompression of c1 and c2 and removing depedencies
>
> ?

Hmm.  ALTER TABLE allows multi comma-separated subcommands, so I don't
think we need to drag VACUUM into this.  The user can just say:

ALTER TABLE tbl ALTER COLUMN c1 SET COMPRESSION awesome, ALTER COLUMN
c2 SET COMPRESSION awesome;

If this is properly integrated into tablecmds.c, that should cause a
single rewrite affecting both columns.