By default, set log_line_prefix = '%m [%p] '.

This value might not be to everyone's taste; in particular, some
people might prefer %t to %m, and others may want %u, %d, or other
fields.  However, it's a vast improvement on the old default of ''.

Christoph Berg

src/backend/utils/misc/guc.c

@@ -3014,7 +3014,7 @@ static struct config_string ConfigureNamesString[] =
 			gettext_noop("If blank, no prefix is used.")
 		},
 		&Log_line_prefix,
-		"",
+		"%m [%p] ",
 		NULL, NULL, NULL
 	},
 

src/backend/utils/misc/postgresql.conf.sample

@@ -430,7 +430,7 @@
 #log_duration = off
 #log_error_verbosity = default		# terse, default, or verbose messages
 #log_hostname = off
-#log_line_prefix = ''			# special values:
+#log_line_prefix = '%m [%p] '		# special values:
 					#   %a = application name
 					#   %u = user name
 					#   %d = database name