How to close idle connections in PostgreSQL automatically?

For those who are interested, here is the solution I came up with, inspired from Craig Ringer's comment:

(...) use a cron job to look at when the connection was last active (see pg_stat_activity) and use pg_terminate_backend to kill old ones.(...)

The chosen solution comes down like this:

• First, we upgrade to Postgresql 9.2.
• Then, we schedule a thread to run every second.
• When the thread runs, it looks for any old inactive connections.

  • A connection is considered inactive if its state is either idle, idle in transaction, idle in transaction (aborted) or disabled.
  • A connection is considered old if its state stayed the same during more than 5 minutes.

• There are additional threads that do the same as above. However, those threads connect to the database with different user.
• We leave at least one connection open for any application connected to our database. (rank() function)

This is the SQL query run by the thread:

WITH inactive_connections AS (
    SELECT
        pid,
        rank() over (partition by client_addr order by backend_start ASC) as rank
    FROM 
        pg_stat_activity
    WHERE
        -- Exclude the thread owned connection (ie no auto-kill)
        pid <> pg_backend_pid( )
    AND
        -- Exclude known applications connections
        application_name !~ '(?:psql)|(?:pgAdmin.+)'
    AND
        -- Include connections to the same database the thread is connected to
        datname = current_database() 
    AND
        -- Include connections using the same thread username connection
        usename = current_user 
    AND
        -- Include inactive connections only
        state in ('idle', 'idle in transaction', 'idle in transaction (aborted)', 'disabled') 
    AND
        -- Include old connections (found with the state_change field)
        current_timestamp - state_change > interval '5 minutes' 
)
SELECT
    pg_terminate_backend(pid)
FROM
    inactive_connections 
WHERE
    rank > 1 -- Leave one connection for each application connected to the database

Connect through a proxy like PgBouncer which will close connections after server_idle_timeout seconds.

If you are using PostgreSQL >= 9.6 there is an even easier solution. Let's suppose you want to delete all idle connections every 5 minutes, just run the following:

alter system set idle_in_transaction_session_timeout='5min';

In case you don't have access as superuser (example on Azure cloud), try:

SET SESSION idle_in_transaction_session_timeout = '5min';

But this latter will work only for the current session, that most likely is not what you want.

To disable the feature,

alter system set idle_in_transaction_session_timeout=0;

or

SET SESSION idle_in_transaction_session_timeout = 0;

(by the way, 0 is the default value).

If you use alter system, you must reload configuration to start the change and the change is persistent, you won't have to re-run the query anymore if, for example, you will restart the server.

To check the feature status:

show idle_in_transaction_session_timeout;