Wikimedia Bugzilla is closed!

I'm currently trying to port one of my Toolserver tools and ran into a problem with DB queries taking orders of magnitude longer on Tools than the TS.

The problem appears to be in queries of the type:
SELECT rc_timestamp FROM recentchanges WHERE rc_user_text='$username' ORDER BY rc_timestamp DESC LIMIT 1

On the Toolserver:
mysql> SELECT rc_timestamp FROM recentchanges WHERE rc_user_text='MaxSem' ORDER BY rc_timestamp DESC LIMIT 1;
+----------------+
| rc_timestamp   |
+----------------+
| 20131009151959 |
+----------------+
1 row in set (0.02 sec)

On Tools:
MariaDB [enwiki_p]> SELECT rc_timestamp FROM recentchanges WHERE rc_user_text='MaxSem' ORDER BY rc_timestamp DESC LIMIT 1;
+----------------+
| rc_timestamp   |
+----------------+
| 20131009151959 |
+----------------+
1 row in set (7.38 sec)

Using rc_user instead makes little difference. The farther in the past the most recent rc_timestamp is, the worse the performance (users with no edits in RC take 10+ seconds). So I'm assuming the problem is an extra index that exists on TS, but not on the Labs replicas.