Re: Proposal - Reduce lock during first phase of VACUUM TRUNCATE from ACCESS EXCLUSIVE to EXCLUSIVE

Hi,

The vacuum truncate operation consists of two phases: a backward scan of
the heap [1] followed by the work to perform the actual truncation [2]. In
the current implementation, both phases maintain an ACCESS EXCLUSIVE lock
on the relation for the duration of the processing. The ACCESS EXCLUSIVE
lock is unnecessarily restrictive during the first phase of VACUUM TRUNCATE
and can prevent read-only access , which can cause extended outages on hot
standby replicas as the primary does not release the lock based on waiting
queries on the hot standby.

I propose modifying the use of an EXCLUSIVE lock during the backward scan
phase, then upgrading that lock to ACCESS EXCLUSIVE only for the actual
truncation phase. Since the truncation phase should be relatively quick,
the impact of the ACCESS EXCLUSIVE lock should be minimal.

For your consideration, on tables with constant high traffic, it may be
impossible to acquire the ACCESS EXCLUSIVE lock needed for the truncation
phase.

I would appreciate feedback on this approach, particularly regarding any
potential issues with changing lock levels mid-operation, the impact on
concurrent operations, and any alternative approaches worth considering.

Thanks,
Ram

[1]
https://github.com/postgres/postgres/blob/master/src/backend/access/heap/vacuumlazy.c#L3267
[2]
https://github.com/postgres/postgres/blob/master/src/backend/access/heap/vacuumlazy.c#L3280