Re: Autovacuum on partitioned table (autoanalyze)

On 2021-Apr-07, Alvaro Herrera wrote:

> However, I just noticed there is a huge problem, which is that the new
> code in relation_needs_vacanalyze() is doing find_all_inheritors(), and
> we don't necessarily have a snapshot that lets us do that.  While adding
> a snapshot acquisition at that spot is a very easy fix, I hesitate to
> fix it that way, because the whole idea there seems quite wasteful: we
> have to look up, open and lock every single partition, on every single
> autovacuum iteration through the database.  That seems bad.  I'm
> inclined to think that a better idea may be to store reltuples for the
> partitioned table in pg_class.reltuples, instead of having to add up the
> reltuples of each partition.  I haven't checked if this is likely to
> break anything.

I forgot to comment on this aspect.  First, I was obviously mistaken
about there not being an active snapshot.  I mean, it's correct that
there isn't.  The issue is that it's really a bug to require that there
is one; it just hasn't failed before because partially detached
partitions aren't very common.  So I patched that as a bug in a
preliminary patch.

Next, the idea of storing the number of tuples in pg_class.reltuples is
a nice one, and I think we should consider it in the long run.  However,
while it can be done as a quick job (shown in the attached, which AFAICT
works fine) there are side-effects -- for example, TRUNCATE doesn't
clear the value, which is surely wrong.  I suspect that if I try to
handle it in this way, it would blow up in some corner case I forgot to
consider.  So, I decided not to go that way, at least for now.

-- 
Álvaro Herrera       Valdivia, Chile