Re: [HACKERS] Custom compression methods

On Mon, Dec 11, 2017 at 8:25 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Mon, Dec 11, 2017 at 7:55 AM, Ildus Kurbangaliev
> <i(dot)kurbangaliev(at)postgrespro(dot)ru> wrote:
> > On Fri, 8 Dec 2017 15:12:42 -0500
> > Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> >> 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.
> >
> > I like this idea, but maybe it should be something like ALTER COLUMN
> > SET COMPRESSION x1 [ PRESERVE x2, x3 ]? 'PRESERVE' is already used in
> > syntax and this syntax will show better which one is current and which
> > ones should be kept.
>
> Sure, that works. And I think pglz should exist in the catalog as a
> predefined, undroppable compression algorithm. So the default for
> each column initially is:
>
> SET COMPRESSION pglz
>
> And if you want to rewrite the table with your awesome custom thing, you
> can do
>
> SET COMPRESSION awesome
>
> But if you want to just use the awesome custom thing for new rows, you can
> do
>
> SET COMPRESSION awesome PRESERVE pglz
>
> Then we can get all the dependencies right, pg_upgrade works, users
> have total control of rewrite behavior, and everything is great. :-)
>

Looks good.

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

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 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

?

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company