如何检查到 Oracle 数据库的最大允许连接数？

I thought this would work, based on this source.

SELECT
'Currently, '
|| (SELECT COUNT(*) FROM V$SESSION)
|| ' out of '
|| DECODE(VL.SESSIONS_MAX,0,'unlimited',VL.SESSIONS_MAX)
|| ' connections are used.' AS USAGE_MESSAGE
FROM
V$LICENSE VL

However, Justin Cave is right. This query gives better results:

SELECT
'Currently, '
|| (SELECT COUNT(*) FROM V$SESSION)
|| ' out of '
|| VP.VALUE
|| ' connections are used.' AS USAGE_MESSAGE
FROM
V$PARAMETER VP
WHERE VP.NAME = 'sessions'

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.

select count(*),sum(decode(status, 'ACTIVE',1,0)) from v$session where type= 'USER'

Note: this only answers part of the question.

If you just want to know the maximum number of sessions allowed, then you can execute in sqlplus, as sysdba:

SQL> show parameter sessions

This gives you an output like:

    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     248
shared_server_sessions               integer

The sessions parameter is the one what you want.

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

Use gv$session for RAC, if you want get the total number of session across the cluster.

v$resource_limit view is so interesting for me in order to glance oracle sessions,processes..:

https://bbdd-error.blogspot.com.es/2017/09/check-sessions-and-processes-limit-in.html