Someone should benchmark that. Unix sockets in my mind should be faster as in theory you have less overhead. But the flow using TCP over loopback might have received many more optimizations over the last decades.
$ pgbench -h localhost -p 5432 -b select-only -T 60 -c 10 -j 2 bench
Password:
pgbench (15.3 (Ubuntu 15.3-1.pgdg22.04+1))
starting vacuum...end.
transaction type: <builtin: select only>
scaling factor: 1
query mode: simple
number of clients: 10
number of threads: 2
maximum number of tries: 1
duration: 60 s
number of transactions actually processed: 10370147
number of failed transactions: 0 (0.000%)
latency average = 0.058 ms
initial connection time = 54.881 ms
tps = 172993.008029 (without initial connection time)
$ pgbench -h /var/run/postgresql -p 5432 -b select-only -T 60 -c 10 -j 2 bench
pgbench (15.3 (Ubuntu 15.3-1.pgdg22.04+1))
starting vacuum...end.
transaction type: <builtin: select only>
scaling factor: 1
query mode: simple
number of clients: 10
number of threads: 2
maximum number of tries: 1
duration: 60 s
number of transactions actually processed: 16890415
number of failed transactions: 0 (0.000%)
latency average = 0.036 ms
initial connection time = 7.186 ms
tps = 281540.260418 (without initial connection time)
YMMV depending on your workload, but Unix sockets should always be significantly faster.
