The command by which the PostgreSQL interactive terminal 'psql' is
called.

If the PostgreSQL binaries are in your system's PATH, the command can
be as simple as 'psql' (on UNIX) or 'psql.exe' (on Windows).

It may be necessary to provide the full filename.

Examples:

    - UNIX: /usr/bin/psql
            /opt/postgresql/bin/psql

    - Windows:
           C:/Program Files/PostgreSQL/8.2/bin/psql.exe
