Check out example codes for "close connections psql". It will help you in understanding the concepts better.

Code Example 1

/* view connections */
SELECT * FROM pg_stat_activity;

/*-— Kill connections*/
SELECT
    pg_terminate_backend(pid)
FROM
    pg_stat_activity
WHERE
    -- don't kill my own connection!
    pid != pg_backend_pid()
    -- don't kill the connections to other databases
    AND datname = 'clinical_data';

/* kill a specific connection */     
select pg_terminate_backend(13337)

Code Example 2

SELECT 
    pg_terminate_backend(pid) 
FROM 
    pg_stat_activity 
WHERE 
    -- don't kill my own connection!
    pid <> pg_backend_pid()
    -- don't kill the connections to other databases
    AND datname = 'database_name'
    ;

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