You can use SHOW:
SHOW max_connections;
This returns the currently effective setting. Be aware that it can differ from the setting in postgresql.conf as there are a multiple ways to set run-time parameters in PostgreSQL. To reset the "original" setting from postgresql.conf in your current session:
RESET max_connections;
However, not applicable to this particular setting. The manual:
This parameter can only be set at server start.
To see all settings:
SHOW ALL;
There is also pg_settings:
The view
pg_settingsprovides access to run-time parameters of the server. It is essentially an alternative interface to theSHOWandSETcommands. It also provides access to some facts about each parameter that are not directly available fromSHOW, such as minimum and maximum values.
For your original request:
SELECT *
FROM pg_settings
WHERE name = 'max_connections';
Finally, there is current_setting(), which can be nested in DML statements:
SELECT current_setting('max_connections');
Related: