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Sync up our various ways of estimating pg_class.reltuples.

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VACUUM thought that reltuples represents the total number of tuples in
the relation, while ANALYZE counted only live tuples.  This can cause
"flapping" in the value when background vacuums and analyzes happen
separately.  The planner's use of reltuples essentially assumes that
it's the count of live (visible) tuples, so let's standardize on having
it mean live tuples.

Another issue is that the definition of "live tuple" isn't totally clear;
what should be done with INSERT_IN_PROGRESS or DELETE_IN_PROGRESS tuples?
ANALYZE's choices in this regard are made on the assumption that if the
originating transaction commits at all, it will happen after ANALYZE
finishes, so we should ignore the effects of the in-progress transaction
--- unless it is our own transaction, and then we should count it.
Let's propagate this definition into VACUUM, too.

Likewise propagate this definition into CREATE INDEX, and into
contrib/pgstattuple's pgstattuple_approx() function.

Tomas Vondra, reviewed by Haribabu Kommi, some corrections by me

Discussion: https://postgr.es/m/16db4468-edfa-830a-f921-39a50498e77e@2ndquadrant.com
This commit is contained in:
Tom Lane
2018-03-22 15:47:29 -04:00
parent cc415a56d0
commit 7c91a0364f
5 changed files with 132 additions and 52 deletions
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4
doc/src/sgml/catalogs.sgml
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@ -1739,8 +1739,8 @@ SCRAM-SHA-256$<replaceable>&lt;iteration count&gt;</replaceable>:<replaceable>&l
<entry><type>float4</type></entry>
<entry></entry>
<entry>
Number of rows in the table. This is only an estimate used by the
planner. It is updated by <command>VACUUM</command>,
Number of live rows in the table. This is only an estimate used by
the planner. It is updated by <command>VACUUM</command>,
<command>ANALYZE</command>, and a few DDL commands such as
<command>CREATE INDEX</command>.
</entry>