From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgresql.org
Subject: Re: VACUUM and ANALYZE disagreeing on what reltuples means
Date: Tue, 25 Jul 2017 13:33:28 +0200
Message-ID: <3d250597-384e-c5f4-7533-b513e366888f@2ndquadrant.com> (raw)
In-Reply-To: <19990.1500936945@sss.pgh.pa.us>
References: <16db4468-edfa-830a-f921-39a50498e77e@2ndquadrant.com>
<19990.1500936945@sss.pgh.pa.us>
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On 7/25/17 12:55 AM, Tom Lane wrote:
> Tomas Vondra <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.
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.patch (text/x-patch, 550B) downloaddiff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.cindex c5c194a..574ca70 100644--- a/src/backend/commands/vacuumlazy.c+++ b/src/backend/commands/vacuumlazy.c@@ -1261,7 +1261,7 @@ 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, tgl@sss.pgh.pa.us
Subject: Re: VACUUM and ANALYZE disagreeing on what reltuples means
In-Reply-To: <3d250597-384e-c5f4-7533-b513e366888f@2ndquadrant.com>
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