Date: Thu, 24 Jan 2019 13:45:49 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Fabien COELHO <coelho@cri.ensmp.fr>,
	PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Subject: Re: PSA: we lack TAP test coverage on NetBSD and OpenBSD
Message-ID: <201901241645.e5syn5lxjldb@alvherre.pgsql>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <11057.1548347725@sss.pgh.pa.us>
User-Agent: NeoMutt/20180716-1013-2c1e88
Precedence: bulk

On 2019-Jan-24, Tom Lane wrote:

> > Also, as 
> > the pseudo-random state is fully controlled, seeded test results are 
> > deterministic so the expected value can be fully checked.
> 
> I found that the "expected value" was different in v11 than HEAD,
> which surprised me.  It looks like the reason is that HEAD sets up
> more/different RandomStates from the same seed than v11 did.  Not
> sure if it's a good thing for this behavior to change across versions.

The rationale behind this was that some internal uses of random numbers
messed up the determinism of user-invoked random functions; 409231919443
commit message says

    While at it, use separate random state for thread administratrivia such
    as deciding which script to run, how long to delay for throttling, or
    whether to log a message when sampling; this not only makes these tasks
    independent of each other, but makes the actual thread run
    deterministic.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services