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  1. Max_connections limit

    Daulat Ram <daulat.ram@exponential.com> — 2019-06-26T07:13:56Z

    Hello team,

We have migrated our database  from Oracle 12c to Postgres 11. I need your suggestions , we have sessions limit in Oracle = 3024 . Do we need to set the same connection limit in Postgres as well. How we can decide the max_connections limit for postgres. Are there any differences in managing connections in Oracle and postgres.

SQL> show parameter sessions;

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
java_max_sessionspace_size           integer     0
java_soft_sessionspace_limit         integer     0
license_max_sessions                 integer     0
license_sessions_warning             integer     0
sessions                             integer     3024
shared_server_sessions               integer
SQL>

Regards,
Daulat

  2. Re: Max_connections limit

    Laurenz Albe <laurenz.albe@cybertec.at> — 2019-06-26T09:05:11Z

    Daulat Ram wrote:
> We have migrated our database  from Oracle 12c to Postgres 11. I need your suggestions ,
> we have sessions limit in Oracle = 3024 . Do we need to set the same connection limit
> in Postgres as well. How we can decide the max_connections limit for postgres.
> Are there any differences in managing connections in Oracle and postgres.

I'd say that is way too high in both Oracle and PostgreSQL.

Set the value to 50 or 100 and get a connection pooler if the
application cannot do that itself.

Yours,
Laurenz Albe
-- 
Cybertec | https://www.cybertec-postgresql.com

  3. Re: Max_connections limit

    Hervé Schweitzer (HER) <herve.schweitzer@dbi-services.com> — 2019-06-26T09:15:45Z

    You now that Postgres don’t have any shared_pool as Oracle, and the  session information ( execution plan, etc..) are only available for the current session. Therefore I also highly recommend to us a connection poll as Laurent wrote, in order to have higher chance that some stuff is already cached in the shared session available. 

Regards
Herve 



Envoyé de mon iPhone

> Le 26 juin 2019 à 11:05, Laurenz Albe <laurenz.albe@cybertec.at> a écrit :
> 
> Daulat Ram wrote:
>> We have migrated our database  from Oracle 12c to Postgres 11. I need your suggestions ,
>> we have sessions limit in Oracle = 3024 . Do we need to set the same connection limit
>> in Postgres as well. How we can decide the max_connections limit for postgres.
>> Are there any differences in managing connections in Oracle and postgres.
> 
> I'd say that is way too high in both Oracle and PostgreSQL.
> 
> Set the value to 50 or 100 and get a connection pooler if the
> application cannot do that itself.
> 
> Yours,
> Laurenz Albe
> -- 
> Cybertec | https://www.cybertec-postgresql.com
> 
> 
>

  4. Re: Max_connections limit

    Rick Otten <rottenwindfish@gmail.com> — 2019-06-26T12:38:46Z

    On Wed, Jun 26, 2019 at 5:16 AM Hervé Schweitzer (HER) <
herve.schweitzer@dbi-services.com> wrote:

> You now that Postgres don’t have any shared_pool as Oracle, and the
> session information ( execution plan, etc..) are only available for the
> current session. Therefore I also highly recommend to us a connection poll
> as Laurent wrote, in order to have higher chance that some stuff is already
> cached in the shared session available.
>
> Regards
> Herve
>
>
The most popular stand-alone connection pooler for PostgreSQL is the oddly
named "pgbouncer":    https://wiki.postgresql.org/wiki/PgBouncer
There are others, of course.

PgPool is also very popular:
https://www.pgpool.net/mediawiki/index.php/Main_Page

Some applications can also manage a connection pool efficiently entirely
within the application itself.

Configuring the maximum number of concurrent connections your database
supports incurs significant overhead in the running database.  New
connections and disconnections also have a high overhead as they occur.  By
moving the connecting/disconnecting logic to a connection pooler you remove
a lot of overhead and load from the database - letting it focus on the
important stuff -- your queries.

It is amazing how many fewer actual connections you need to the database
when you configure a pooler.  Most connections from applications and users
are idle most of the time.   Even on busy web servers.  They just keep that
pathway open in case they need to run a query to save on the overhead of
having to open a new one every time.   By using a pooler you only need to
configure connections for the number of concurrent _queries_ rather than
concurrent application and user open but idle connections.

  5. RE: Max_connections limit

    Igor Neyman <ineyman@perceptron.com> — 2019-06-26T14:29:14Z

    From: Daulat Ram [mailto:Daulat.Ram@exponential.com]
Sent: Wednesday, June 26, 2019 3:14 AM
To: pgsql-performance@lists.postgresql.org
Subject: Max_connections limit

Hello team,

We have migrated our database  from Oracle 12c to Postgres 11. I need your suggestions , we have sessions limit in Oracle = 3024 . Do we need to set the same connection limit in Postgres as well. How we can decide the max_connections limit for postgres. Are there any differences in managing connections in Oracle and postgres.

SQL> show parameter sessions;

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
java_max_sessionspace_size           integer     0
java_soft_sessionspace_limit         integer     0
license_max_sessions                 integer     0
license_sessions_warning             integer     0
sessions                             integer     3024
shared_server_sessions               integer
SQL>

Regards,
Daulat


The difference between Oracle and PG is that Oracle has "built-in" connection pooler, and PG does not.
You should use external pooler (i.e. PgBouncer) and reduce number of allowed connections in PG config to about 50, while allowing thousands client connection when configuring PgBouncer.

Regards,
Igor Neyman