libpq is reentrant and thread-safe by default.
   You might need to use special compiler command-line
   options when you compile your application code.  Refer to your
   system's documentation for information about how to build
   thread-enabled applications, or look in
   src/Makefile.global for PTHREAD_CFLAGS
   and PTHREAD_LIBS.  This function allows the querying of
   libpq's thread-safe status:
  
   One thread restriction is that no two threads attempt to manipulate
   the same PGconn object at the same time. In particular,
   you cannot issue concurrent commands from different threads through
   the same connection object. (If you need to run concurrent commands,
   use multiple connections.)
  
   PGresult objects are normally read-only after creation,
   and so can be passed around freely between threads.  However, if you use
   any of the PGresult-modifying functions described in
   Section 34.11 or Section 34.13, it's up
   to you to avoid concurrent operations on the same PGresult,
   too.
  
   The deprecated functions PQrequestCancel and
   PQoidStatus are not thread-safe and should not be
   used in multithread programs.  PQrequestCancel
   can be replaced by PQcancel.
   PQoidStatus can be replaced by
   PQoidValue.
  
   If you are using Kerberos inside your application (in addition to inside
   libpq), you will need to do locking around
   Kerberos calls because Kerberos functions are not thread-safe.  See
   function PQregisterThreadLock in the
   libpq source code for a way to do cooperative
   locking between libpq and your application.