From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Haribabu Kommi <kommi.haribabu@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: VACUUM and ANALYZE disagreeing on what reltuples means
Date: Sun, 24 Sep 2017 20:39:03 +0200
Message-ID: <94f58897-861b-accf-a9f1-af4be816c0d3@2ndquadrant.com> (raw)
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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
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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http://www.postgresql.org/mailpref/pgsql-hackersAttachments:vacuum-reltuples-fix-v2.patch (text/x-patch, 1.3K) downloaddiff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.cindex 45b1859..1886f0d 100644--- a/src/backend/commands/vacuumlazy.c+++ b/src/backend/commands/vacuumlazy.c@@ -198,7 +198,6 @@ lazy_vacuum_rel(Relation onerel, int options, VacuumParams *params,
BlockNumber new_rel_pages;
double new_rel_tuples;
BlockNumber new_rel_allvisible;
- double new_live_tuples;
TransactionId new_frozen_xid;
MultiXactId new_min_multi;
@@ -335,13 +334,9 @@ lazy_vacuum_rel(Relation onerel, int options, VacuumParams *params,
false);
/* report results to the stats collector, too */
- new_live_tuples = new_rel_tuples - vacrelstats->new_dead_tuples;- if (new_live_tuples < 0)- new_live_tuples = 0; /* just in case */-
pgstat_report_vacuum(RelationGetRelid(onerel),
onerel->rd_rel->relisshared,
- new_live_tuples,+ new_rel_tuples,
vacrelstats->new_dead_tuples);
pgstat_progress_end_command();
@@ -1267,7 +1262,7 @@ lazy_scan_heap(Relation onerel, int options, LVRelStats *vacrelstats,
vacrelstats->new_rel_tuples = vac_estimate_reltuples(onerel, false,
nblocks,
vacrelstats->tupcount_pages,
- num_tuples);+ num_tuples - nkeep);
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To: pgsql-hackers@postgresql.org
Cc: tomas.vondra@2ndquadrant.com, kommi.haribabu@gmail.com, tgl@sss.pgh.pa.us
Subject: Re: VACUUM and ANALYZE disagreeing on what reltuples means
In-Reply-To: <94f58897-861b-accf-a9f1-af4be816c0d3@2ndquadrant.com>
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