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

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.