How to check the maximum number of allowed connections to an Oracle database?

Solution 1:

There are a few different limits that might come in to play in determining the number of connections an Oracle database supports. The simplest approach would be to use the SESSIONS parameter and V$SESSION, i.e.

The number of sessions the database was configured to allow

SELECT name, value 
  FROM v$parameter
 WHERE name = 'sessions'

The number of sessions currently active

SELECT COUNT(*)
  FROM v$session

As I said, though, there are other potential limits both at the database level and at the operating system level and depending on whether shared server has been configured. If shared server is ignored, you may well hit the limit of the PROCESSES parameter before you hit the limit of the SESSIONS parameter. And you may hit operating system limits because each session requires a certain amount of RAM.

Solution 2:

The sessions parameter is derived from the processes parameter and changes accordingly when you change the number of max processes. See the Oracle docs for further info.

To get only the info about the sessions:

    select current_utilization, limit_value 
    from v$resource_limit 
    where resource_name='sessions';

CURRENT_UTILIZATION LIMIT_VALUE
------------------- -----------
                110         792

Try this to show info about both:

    select resource_name, current_utilization, max_utilization, limit_value 
    from v$resource_limit 
    where resource_name in ('sessions', 'processes');

RESOURCE_NAME CURRENT_UTILIZATION MAX_UTILIZATION LIMIT_VALUE
------------- ------------------- --------------- -----------
processes                      96             309         500
sessions                      104             323         792