BUG#12542: Added comments

sql/log_event.cc:
  Added comments
sql/sql_parse.cc:
  Added comments

sql/log_event.cc

@@ -1008,6 +1008,16 @@ int Query_log_event::exec_event(struct st_relay_log_info* rli)
 #endif
   clear_all_errors(thd, rli);
 
+  /*
+    Note:   We do not need to execute reset_one_shot_variables() if this
+            db_ok() test fails.
+    Reason: The db stored in binlog events is the same for SET and for
+            its companion query.  If the SET is ignored because of
+            db_ok(), the companion query will also be ignored, and if
+            the companion query is ignored in the db_ok() test of
+            ::exec_event(), then the companion SET also have so we
+            don't need to reset_one_shot_variables().
+  */
   if (db_ok(thd->db, replicate_do_db, replicate_ignore_db))
   {
     thd->set_time((time_t)when);
@@ -1762,6 +1772,16 @@ int Load_log_event::exec_event(NET* net, struct st_relay_log_info* rli,
     Create_file_log_event::exec_event() and then discarding Append_block and
     al. Another way is do the filtering in the I/O thread (more efficient: no
     disk writes at all).
+
+
+    Note:   We do not need to execute reset_one_shot_variables() if this
+            db_ok() test fails.
+    Reason: The db stored in binlog events is the same for SET and for
+            its companion query.  If the SET is ignored because of
+            db_ok(), the companion query will also be ignored, and if
+            the companion query is ignored in the db_ok() test of
+            ::exec_event(), then the companion SET also have so we
+            don't need to reset_one_shot_variables().
   */
   if (db_ok(thd->db, replicate_do_db, replicate_ignore_db))
   {

sql/sql_parse.cc

@@ -1998,7 +1998,8 @@ mysql_execute_command(THD *thd)
 
       Exceptions are:
       - SET: we always execute it (Not that many SET commands exists in
-        the binary log anyway)
+        the binary log anyway -- only 4.1 masters write SET statements,
+	in 5.0 there are no SET statements in the binary log)
       - DROP TEMPORARY TABLE IF EXISTS: we always execute it (otherwise we
         have stale files on slave caused by exclusion of one tmp table).
     */