Re: [HACKERS] VACUUM and ANALYZE disagreeing on what reltuples means

> On 06 Sep 2017, at 09:45, Haribabu Kommi <kommi.haribabu@gmail.com> wrote:
>
> On Tue, Jul 25, 2017 at 9:33 PM, Tomas Vondra <tomas.vondra@2ndquadrant.com <mailto:tomas.vondra@2ndquadrant.com>>
wrote:
> On 7/25/17 12:55 AM, Tom Lane wrote:
> Tomas Vondra <tomas.vondra@2ndquadrant.com <mailto:tomas.vondra@2ndquadrant.com>> writes:
> It seems to me that VACUUM and ANALYZE somewhat disagree on what
> exactly reltuples means. VACUUM seems to be thinking that reltuples
> = live + dead while ANALYZE apparently believes that reltuples =
> live
>
> The question is - which of the reltuples definitions is the right
> one? I've always assumed that "reltuples = live + dead" but perhaps
> not?
>
> 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
tuplesas 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
needto run ANALYZE in the other session. 
>
> Thanks for the patch.
> From the mail, I understand that this patch tries to improve the
> reltuples value update in the catalog table by the vacuum command
> to consider the proper visible tuples similar like analyze command.
>
> -                                                         num_tuples);
> +                                                         num_tuples - nkeep);
>
> With the above correction, there is a problem in reporting the number
> of live tuples to the stats.
>
> postgres=# select reltuples, n_live_tup, n_dead_tup
>               from pg_stat_user_tables join pg_class using (relname)
>              where relname = 't';
>  reltuples | n_live_tup | n_dead_tup
> -----------+------------+------------
>     899818 |     799636 |     100182
> (1 row)
>
>
> The live tuples data value is again decremented with dead tuples
> value before sending them to stats in function lazy_vacuum_rel(),
>
>     /* report results to the stats collector, too */
>     new_live_tuples = new_rel_tuples - vacrelstats->new_dead_tuples;
>
> The fix needs a correction here also. Or change the correction in
> lazy_vacuum_rel() function itself before updating catalog table similar
> like stats.

This patch is marked Waiting for Author, have you had a chance to look at this
to address the comments in the above review?

cheers ./daniel

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