Re: [HACKERS] Custom compression methods

On Wed, Mar 10, 2021 at 03:50:48PM -0500, Robert Haas wrote:
> On Wed, Mar 10, 2021 at 6:52 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > The pending comment is providing a way to rewrite a table and
> > re-compress the data with the current compression method.
>
> I spent some time poking at this yesterday and ran couldn't figure out
> what was going on here. There are two places where we rewrite tables.
> One is the stuff in cluter.c, which handles VACUUM FULL and CLUSTER.
> That eventually calls reform_and_rewrite_tuple(), which deforms the
> old tuple and creates a new one, but it doesn't seem like there's
> anything in there that would expand toasted values, whether external
> or inline compressed. But I think that can't be right, because it
> seems like then you'd end up with toast pointers into the old TOAST
> relation, not the new one, which would cause failures later. So I must
> be missing something here.

I did this the empirical way.

postgres=# CREATE TABLE t (a text compression lz4);
postgres=# INSERT INTO t SELECT repeat('a',99999);
postgres=# ALTER TABLE t ALTER a SET COMPRESSION pglz;
postgres=# VACUUM FULL t;
postgres=# SELECT pg_column_compression(a) FROM t;
pg_column_compression | lz4

Actually, a --without-lz4 build can *also* VACUUM FULL the lz4 table.

It looks like VACUUM FULL t processes t but not its toast table (which is
strange to me, since VACUUM processes both, but then autovacuum also processes
them independently).

--
Justin