How to install an extension in a postgreSQL database

Are the desired extensions available in our installation?

[postgres@myserver.no ~]$ echo "SELECT * FROM pg_available_extensions;" | psql | egrep 'cube|earthdistance';
 cube               | 1.5             |                   | data type for multidimensional cubes
 earthdistance      | 1.1             |                   | calculate great-circle distances on the surface of the Earth

Yes they are.

Source : https://www.postgresql.org/docs/current/view-pg-available-extensions.html

Connect to the relevant database and create the extensions in the schema you want them:

postgres=# \connect mydb1
You are now connected to database "mydb1" as user "postgres".
mydb1=# create extension if not exists cube with schema myschema1 cascade;
CREATE EXTENSION
mydb1=# create extension if not exists earthdistance  with schema myschema1 cascade;
CREATE EXTENSION
mydb1=# exit

Check if they have indeed been installed:

[postgres@myserver.no ~]$ psql mydb01
psql (15.4)
Type "help" for help.

sit1=# \dx
                                                List of installed extensions
        Name        | Version |       Schema        |                              Description
--------------------+---------+---------------------+------------------------------------------------------------------------
 cube               | 1.5     | myschema1           | data type for multidimensional cubes
 earthdistance      | 1.1     | myschema1           | calculate great-circle distances on the surface of the Earth
 pg_qualstats       | 2.0.4   | public              | An extension collecting statistics about quals
 pg_stat_statements | 1.10    | public              | track planning and execution statistics of all SQL statements executed
 plpgsql            | 1.0     | pg_catalog          | PL/pgSQL procedural language
(5 rows)

Create extension is documented here