Re: VACUUM and ANALYZE disagreeing on what reltuples means

On Tue, Jul 25, 2017 at 07:02:28PM +0200, Tomas Vondra wrote:
> On 7/25/17 5:04 PM, Tom Lane wrote:
> >Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
> >>Attached is a patch that (I think) does just that. The disagreement
> >>was caused by VACUUM treating recently dead tuples as live, while
> >>ANALYZE treats both of those as dead.
> >
> >>At first I was worried that this will negatively affect plans in
> >>the long-running transaction, as it will get underestimates (due
> >>to reltuples not including rows it can see). But that's a problem
> >>we already have anyway, you just need to run ANALYZE in the other
> >>session.
> >
> >This definitely will have some impact on plans, at least in cases
> >where there's a significant number of unvacuumable dead tuples. So I
> >think it's a bit late for v10, and I wouldn't want to back-patch at
> >all. Please add to the next commitfest.
> >
>
> I dare to disagree here, for two reasons.
>
> Firstly, the impact *is* already there, it only takes running ANALYZE. Or
> VACUUM ANALYZE. In both those cases we already end up with
> reltuples=n_live_tup.
>
> Secondly, I personally strongly prefer stable predictable behavior over
> intermittent oscillations between two values. That's a major PITA on
> production, both to investigate and fix.

> FWIW I personally see this as a fairly annoying bug, and would vote to
> backpatch it, although I understand people might object.

I tend to agree. If you have a setup that somehow never uses ANALYZE or never
uses VACUUM on a particular table, you might prefer today's (accidental)
behavior. However, the discrepancy arises only on a table that gets dead
tuples, and a table that gets dead tuples will see both VACUUM and ANALYZE
soon enough. It does seem like quite a stretch to imagine someone wanting
plans to depend on which of VACUUM or ANALYZE ran most recently.