Subject: Re: VACUUM and ANALYZE disagreeing on what reltuples means
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgresql.org
References: <16db4468-edfa-830a-f921-39a50498e77e@2ndquadrant.com>
 <19990.1500936945@sss.pgh.pa.us>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <3d250597-384e-c5f4-7533-b513e366888f@2ndquadrant.com>
Date: Tue, 25 Jul 2017 13:33:28 +0200
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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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 name="vacuum-reltuples-fix.patch"
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diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index 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);
 
 	/*
 	 * Release any remaining pin on visibility map page.

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