Using psql how do I list extensions installed in a database?

Question

How do I list all extensions that are already installed in a database or schema from psql?

See also

• Finding a list of available extensions that PostgreSQL ships with

Answer 1

In psql that would be

\dx 

See the manual of psql for details.

Doing it in plain SQL it would be a select on pg_extension:

SELECT *  FROM pg_extension;