How to drop all connections to a specific database without stopping the server?

The query like this should help (assuming the database is named 'db'):

select pg_terminate_backend(pid) from pg_stat_activity where datname='db';

pid used to be called procpid, so if you're using a version of postgres older than 9.2 you could try the following:

select pg_terminate_backend(procpid) from pg_stat_activity where datname='db';

However you have to be a superuser to disconnect other users.

It might also be useful to REVOKE CONNECT ON DATABASE FROM PUBLIC or something similar, and then GRANT it afterward.

Here is my answer to very similar question on StackOverflow.

Depending on your version of postgresql you might run into a bug, that makes pg_stat_activity to omit active connections from dropped users. These connections are also not shown inside pgAdminIII.

If you are doing automatic testing (in which you also create users) this might be a probable scenario.

In this case you need to revert to queries like:

 SELECT pg_terminate_backend(pg_stat_activity.procpid) 
 FROM pg_stat_get_activity(NULL::integer) 
 WHERE datid=(SELECT oid from pg_database where datname = 'your_database');

This can be used to "free" a database from client connections, so that you for example can rename it:

SELECT pg_terminate_backend(procpid) FROM pg_stat_activity WHERE datname='current_db';
ALTER DATABASE current_db RENAME TO old_db;
ALTER DATABASE new_db RENAME TO current_db;

Be aware that this might cause problematic behaviour to your client apps. Data actualy should not be currupted due to using transactions.