Re: Modest proposal to extend TableAM API for controlling cluster commands

On Wed, Jun 15, 2022 at 8:18 PM Andres Freund <andres(at)anarazel(dot)de> wrote:

> > If a simple callback like
> > relation_supports_cluster(Relation rel) is too simplistic
>

Seems like it should be called:
relation_supports_compaction[_by_removal_of_interspersed_dead_tuples]

Basically, if the user tells the table to make itself smaller on disk by
removing dead tuples, should we support the case where the Table AM says:
"Sorry, I cannot do that"?

If yes, then naming the table explicitly should elicit an error. Having
the table chosen implicitly should provoke a warning. For ALTER TABLE
CLUSTER there should be an error: which makes the implicit CLUSTER command
a non-factor.

However, given that should the table structure change it is imperative that
the Table AM be capable of producing a new physical relation with the
correct data, which will have been compacted as a side-effect, it seems
like, explicit or implicit, expecting any Table AM to do that when faced
with Vacuum Full is reasonable. Which leaves deciding how to allow a table
with a given TAM to prevent itself from being added to the CLUSTER roster.
And decide whether an opt-out feature for implicit VACUUM FULL is something
we should offer as well.

I'm doubtful that a TAM that is pluggable into the MVCC and WAL
architecture of PostgreSQL could avoid this basic contract between the
system and its users.

David J.