Re: VACUUM and ANALYZE disagreeing on what reltuples means

On 09/06/2017 09:45 AM, Haribabu Kommi 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 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.
> 
> 
> 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.
> 

Ah, haven't noticed that for some reason - you're right, we estimate the
reltuples based on (num_tuples - nkeep), so it doesn't make much sense
to subtract nkeep again. Attached is v2 of the fix.

I've removed the subtraction from lazy_vacuum_rel(), leaving just

    new_live_tuples = new_rel_tuples;

and now it behaves as expected (no second subtraction). That means we
can get rid of new_live_tuples altogether (and the protection against
negative values), and use new_rel_tuples directly.

Which pretty much means that in this case

    (pg_class.reltuples == pg_stat_user_tables.n_live_tup)

but I guess that's fine, based on the initial discussion in this thread.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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