Conditions to be connected through UNIX-sockets ?

Hello,

In order to improve the speed, I would like to know the conditions to be
connected through unix sockets (faster) instead tcp sockets (slower).

If I specify only the database name (with or without login/password), I
assume that I will be connect to the PostgreSQL through Unix Sockets.

What will be used (unix or tcpip sockets) if :

- I set a specific port (5433, for example) but not hostname ?
- I set the localhost hostname or the 127.0.0.1 ip ?
- I set the 'external' IP of the machine (ex. 192.168.0.20) and that I
make the call from the same machine ?

I've looked into the documentation and I didn't find the cases that will
bring to the use of Unix-Sockets or Tcpip-Sockets... When I will receive
the answers, I will write a little text in order to complete the
documentation. :-)

Thanks in advance ! :-)

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Bruno BAGUETTE - pgsql-ml(at)baguette(dot)net