On Wed, Jun 15, 2022 at 8:18 PM Andres Freund <andres@anarazel.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.
