Re: Slow execution of SET ROLE, SET search_path and RESET ROLE

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