From: Tom Lane <tgl@sss.pgh.pa.us>
To: =?UTF-8?Q?Ulf_Lohbr=C3=BCgge?= <ulf.lohbruegge@gmail.com>
cc: Scott Marlowe <scott.marlowe@gmail.com>,
        Andres Freund <andres@anarazel.de>,
        "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: [PERFORM] Slow execution of SET ROLE,
 SET search_path and RESET ROLE
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Comments: In-reply-to =?UTF-8?Q?Ulf_Lohbr=C3=BCgge?=
 <ulf.lohbruegge@gmail.com>
	message dated "Thu, 07 Dec 2017 13:54:15 +0100"
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Date: Thu, 07 Dec 2017 11:01:13 -0500
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=?UTF-8?Q?Ulf_Lohbr=C3=BCgge?= <ulf.lohbruegge@gmail.com> writes:
> I could reproduce part of the things I described earlier in this thread. A
> guy named Andriy Senyshyn mailed me after reading this thread here (he
> could somehow not join the mailing list) and observed a difference when
> issuing "SET ROLE" as user postgres and as a non-superuser.

This isn't particularly surprising in itself.  When we know that the
session user is a superuser, SET ROLE just succeeds immediately.
Otherwise we have to determine whether the SET is allowed, ie, is
the session user a member of the specified role.

It looks like the first time such a question is asked within a session,
we build and cache a list of all the roles the session user is a member
of (directly or indirectly).  That's what's taking the time here ---
apparently in your test case, the "admin" role is a member of a whole lot
of roles?

			regards, tom lane