Horton

Re: VACUUM and ANALYZE disagreeing on what reltuples means

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: pgsql-hackers@postgresql.org
Date: 2017-07-25T15:04:03Z
Lists: pgsql-hackers 
Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
> On 7/25/17 12:55 AM, Tom Lane wrote:
>> I think the planner basically assumes that reltuples is the live
>> tuple count, so maybe we'd better change VACUUM to get in step.

> 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.

			regards, tom lane

Commits

  1. Sync up our various ways of estimating pg_class.reltuples.

  2. Fix tuple counting in SP-GiST index build.

  3. Fix errors in contrib/bloom index build.

  4. When updating reltuples after ANALYZE, just extrapolate from our sample.

  5. Avoid holding AutovacuumScheduleLock while rechecking table statistics.