Re: reloption to prevent VACUUM from truncating empty pages at the end of relation

Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com> writes:
> What if we remember the buffers as seen by count_nondeletable_pages() and
> then just discard those specific buffers instead of scanning the entire
> shared_buffers again?

That's an idea.

> Surely we revisit all to-be-truncated blocks before
> actual truncation. So we already know which buffers to discard. And we're
> holding exclusive lock at that point, so nothing can change underneath. Of
> course, we can't really remember a large number of buffers, so we can do
> this in small chunks.

Hm? We're deleting the last N consecutive blocks, so it seems like we
just need to think in terms of clearing that range. I think this can
just be a local logic change inside DropRelFileNodeBuffers().

You could optimize it fairly easily with some heuristic that compares
N to sizeof shared buffers; if it's too large a fraction, the existing
implementation will be cheaper than a bunch of hashtable probes.

regards, tom lane