Date: Tue, 7 Nov 2017 07:11:08 -0800
From: Andres Freund <andres@anarazel.de>
To: Ulf =?iso-8859-1?Q?Lohbr=FCgge?= <ulf.lohbruegge@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Slow execution of SET ROLE, SET search_path and RESET
 ROLE
Message-ID: <20171107151108.3jtmta4xrgmuyrqf@alap3.anarazel.de>
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Hi,

On 2017-11-07 11:11:36 +0100, Ulf Lohbrügge wrote:
> I'm using PostgreSQL 9.5.9 on Debian and experience slow execution of some
> basic SET statements.
> 
> I created about 1600 roles and use that setup for a multi tenancy
> application:

Hm. How often do you drop/create these roles?  How many other
roles/groups is one role a member of?


> My application solely uses the role 'admin' to connect to the database.
> When performing sql statements for a specific tenant (e.g. tenant1337), a
> connection with user 'admin' is established and the following commands are
> executed:
> 
> SET ROLE 1337;
> SET search_path = tenant1337;
> 
> Then the application uses that connection to perform various statements in
> the database.

Just to be sure: You realize bad application code could escape from
that, right?


> My application is a web service that approximately executes some hundred
> statements per second.
> 
> I set "log_min_duration_statement = 200ms" and I get about 2k to 4k lines
> per day with statements like "SET ROLE"", "SET search_path ..." and "RESET
> ROLE":
> 
> --snip--
> 2017-11-07 09:44:30 CET [27760]: [3-1] user=admin,db=mydb LOG:  duration:
> 901.591 ms  execute <unnamed>: SET ROLE "tenant762"
> 2017-11-07 09:44:30 CET [27659]: [4-1] user=admin,db=mydb LOG:  duration:
> 1803.971 ms  execute <unnamed>: SET ROLE "tenant392"

That is weird.


> Besides those peaks in statement duration, my application performs (i.e.
> has acceptable response times) most of the time.
> 
> Is there anything I can do to improve performance here?
> Any help is greatly appreciated!

Can you manually reproduce the problem? What times do you get if you
manually run the statement?

Greetings,

Andres Freund


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