Re: ALTER TABLE .. DETACH PARTITION CONCURRENTLY

On 2021-Apr-23, Alvaro Herrera wrote:

> I think the patch I posted was too simple. I think a real fix requires
> us to keep track of exactly in what way the partdesc is outdated, so
> that we can compare to the current situation in deciding to use that
> partdesc or build a new one. For example, we could keep track of a list
> of OIDs of detached partitions (and it simplifies things a lot if allow
> only a single partition in this situation, because it's either zero OIDs
> or one OID); or we can save the Xmin of the pg_inherits tuple for the
> detached partition (and we can compare that Xmin to our current active
> snapshot and decide whether to use that partdesc or not).
>
> I'll experiment with this a little more and propose a patch later today.

This (POC-quality) seems to do the trick.

(I restored the API of find_inheritance_children, because it was getting
a little obnoxious. I haven't thought this through but I think we
should do something like it.)

> I don't think it's too much of a problem to state that you need to
> finalize one detach before you can do the next one. After all, with
> regular detach, you'd have the tables exclusively locked so you can't do
> them in parallel anyway. (It also increases the chances that people
> will finalize detach operations that went unnoticed.)

I haven't added a mechanism to verify this; but with asserts on, this
patch will crash if you have more than one. I think the behavior is not
necessarily sane with asserts off, since you'll get an arbitrary
detach-Xmin assigned to the partdesc, depending on catalog scan order.

--
Álvaro Herrera Valdivia, Chile
"El hombre nunca sabe de lo que es capaz hasta que lo intenta" (C. Dickens)