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I wonder if the UNIX domain socket connections with postgresql are faster then tcp connections from localhost in high concurrency rate and if it does, by how much?
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A UNIX socket is an inter-process communication mechanism that allows bidirectional data exchange between processes running on the same machine. IP sockets (especially TCP/IP sockets) are a mechanism allowing communication between processes over the network.
Unix domain sockets are often twice as fast as a TCP socket when both peers are on the same host. The Unix domain protocols are not an actual protocol suite, but a way of performing client/server communication on a single host using the same API that is used for clients and servers on different hosts.
Valid socket types in the UNIX domain are: SOCK_STREAM, for a stream-oriented socket; SOCK_DGRAM, for a datagram-oriented socket that preserves message boundaries (as on most UNIX implementations, UNIX domain datagram sockets are always reliable and don't reorder datagrams); and (since Linux 2.6.
A Unix domain socket aka UDS or IPC socket (inter-process communication socket) is a data communications endpoint for exchanging data between processes executing on the same host operating system.
Postgres core developer Bruce Momjian has blogged about this topic. Momjian states, "Unix-domain socket communication is measurably faster." He measured query network performance showing that the local domain socket was 33% faster than using the TCP/IP stack.
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UNIX domain sockets should offer better performance than TCP sockets over loopback interface (less copying of data, fewer context switches), but I don't know whether the performance increase can be demonstrated with PostgreSQL.
I found a small comparison on the FreeBSD mailinglist: http://lists.freebsd.org/pipermail/freebsd-performance/2005-February/001143.html.
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