Re: [HACKERS] Custom compression methods

On Wed, Oct 21, 2020 at 8:51 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Thu, Oct 8, 2020 at 5:54 PM Tomas Vondra
> <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> > And is the oidvector actually needed? If we have the extra catalog,
> > can't we track this simply using the regular dependencies? So we'd have
> > the attcompression OID of the current compression method, and the
> > preserved values would be tracked in pg_depend.
>
> If we go that route, we have to be sure that no such dependencies can
> exist for any other reason. Otherwise, there would be confusion about
> whether the dependency was there because values of that type were
> being preserved in the table, or whether it was for the hypothetical
> other reason. Now, admittedly, I can't quite think how that would
> happen. For example, if the attribute default expression somehow
> embedded a reference to a compression AM, that wouldn't cause this
> problem, because the dependency would be on the attribute default
> rather than the attribute itself. So maybe it's fine.

Yeah, and moreover in the new patchset, we are storing the compression
methods in the new catalog 'pg_compression' instead of merging with
the pg_am. So I think only for the preserve purpose we will maintain
the attribute -> pg_compression dependency so it should be fine.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com