From: Tom Lane <tgl@sss.pgh.pa.us>
To: Simon Riggs <simon@2ndquadrant.com>
cc: pgsql-committers@lists.postgresql.org,
        Konstantin Knizhnik <k.knizhnik@postgrespro.ru>
Subject: Re: pgsql: Allow HOT updates for some expression indexes
In-reply-to: <11787.1522184835@sss.pgh.pa.us>
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Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Tue, 27 Mar 2018 17:07:15 -0400"
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Date: Tue, 27 Mar 2018 17:26:26 -0400
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I wrote:
> The test script appears to imagine that pgstats outputs update
> instantaneously.

Oh, wait, it's looking at pg_stat_xact, which is the *local*, unsent
stats information.  So your problem is actually the reverse of that:
if the test runs too slowly, it fails, because at some point the
unsent stats information will get flushed out to the collector and
disappear from the pg_stat_xact view.

You could probably make this more reliable by wrapping each test
stanza in a transaction, ie instead of

create table keyvalue ...;
... do something to table ...
select pg_stat_get_xact_tuples_hot_updated('keyvalue'::regclass);
drop table keyvalue;

do

begin;
create table keyvalue ...;
... do something to table ...
select pg_stat_get_xact_tuples_hot_updated('keyvalue'::regclass);
rollback;

(no need for a DROP if you're rolling it all back)

			regards, tom lane