Fix performance regression in dblink connection speed.

Previous commit e5de601267 modified dblink
to ensure client encoding matched the server. However the added
PQsetClientEncoding() call added significant overhead. Restore original
performance in the common case where client encoding already matches
server encoding by doing nothing in that case. Applies to all active
branches.

Issue reported and work sponsored by Zonar Systems.

contrib/dblink/dblink.c

@@ -192,7 +192,8 @@ typedef struct remoteConnHashEnt
 							 errdetail_internal("%s", msg))); \
 				} \
 				dblink_security_check(conn, rconn); \
-				PQsetClientEncoding(conn, GetDatabaseEncodingName()); \
+				if (PQclientEncoding(conn) != GetDatabaseEncoding()) \
+					PQsetClientEncoding(conn, GetDatabaseEncodingName()); \
 				freeconn = true; \
 			} \
 	} while (0)
@@ -271,8 +272,9 @@ dblink_connect(PG_FUNCTION_ARGS)
 	/* check password actually used if not superuser */
 	dblink_security_check(conn, rconn);
 
-	/* attempt to set client encoding to match server encoding */
-	PQsetClientEncoding(conn, GetDatabaseEncodingName());
+	/* attempt to set client encoding to match server encoding, if needed */
+	if (PQclientEncoding(conn) != GetDatabaseEncoding())
+		PQsetClientEncoding(conn, GetDatabaseEncodingName());
 
 	if (connname)
 	{