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

doc/src/sgml/config.sgml

@@ -5004,7 +5004,8 @@ local0.*    /var/log/postgresql
          value will pad on the left. Padding can be useful to aid human
          readability in log files.
          This parameter can only be set in the <filename>postgresql.conf</>
-         file or on the server command line. The default is an empty string.
+         file or on the server command line. The default is
+         <literal>'%m [%p] '</> which logs a time stamp and the process ID.
 
          <informaltable>
           <tgroup cols="3">
@@ -5142,6 +5143,17 @@ FROM pg_stat_activity;
          include those escapes if you are logging to <application>syslog</>.
         </para>
        </tip>
+
+       <tip>
+        <para>
+         The <literal>%q</> escape is useful when including information that is
+         only available in session (backend) context like user or database
+         name.  For example:
+<programlisting>
+log_line_prefix = '%m [%p] %q%u@%d/%a '
+</programlisting>
+        </para>
+       </tip>
       </listitem>
      </varlistentry>
 

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