Re: psql - add special variable to reflect the last query status

From: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
To: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, PostgreSQL Developers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: psql - add special variable to reflect the last query status
Date: 2017-09-07 20:12:51
Message-ID: alpine.DEB.2.20.1709072148300.3598@lancre
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Hello Pavel,

> I checked performance - the most fast queries are execution of simple
> prepared statement
>
> prepare x as select 1;
> -- 1000000 x
> execute x;
> execute x;
> execute x;
> execute x;
>
> ## patched
> [pavel(at)nemesis postgresql]$ time psql -At -1 postgres -f ~/xxx.sql >
> /dev/null
>
> real 0m44,887s
> user 0m11,703s
> sys 0m6,942s
>
> This is probably the most worst case, what is possible and see some
> slowdown - in this case there is about 10% slowdown -
>
> but it is pretty untypical - the one query was less than 50 microsec. When
> there will be any IO activity or network usage, than this patch doesn't
> create any significant overhead.

Interesting. Thanks for the test.

I tried to replicate with a variant without any output: "SELECT;"

SELECT NOW() AS start \gset
BEGIN;
SELECT; -- 2^19 times
END;
SELECT NOW() - :'start';

The run time is about 19 µs per SELECT on my laptop. Over 33 runs each
alternating master with and without the patch, I got the following stats
on the measured time in seconds (average, stddev [min Q1 median Q3 max]):

- with : 9.898 ± 0.158 [9.564, 9.762, 9.936, 10.037, 10.108]
- without: 9.419 ± 0.294 [8.670, 9.226, 9.533, 9.625, 9.845]

This seems consistent and significant. It suggests a 0.40-0.50 s
difference, that is about 5%, i.e. about (under) 1 µs overhead per
statement in pretty defavorable circumstances.

--
Fabien.

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