Simple queries hang on "statistics"

[Jay Paroline]

Hello,

We've recently had a strange issue where simple selects seem to be hung up on the statistics state, which means MySQL is calculating an execution plan, but why does that cause these queries to hang, they are literally just direct lookups in most cases.

| 501261041 | user | ip:54204 | gshark_db | Query | 0 | statistics | SELECT SessionID, SessionData, SessionTime, TSAdded FROM Sessions WHERE SessionString = '(26 char session id)'|
| 501261042 | user | ip:46137 | gshark_db | Query | 0 | statistics | SELECT SessionID, SessionData, SessionTime, TSAdded FROM Sessions WHERE SessionString = '(26 char session id)'|
| 501261043 | user | ip:50723 | gshark_db | Query | 0 | statistics | SELECT SessionID, SessionData, SessionTime, TSAdded FROM Sessions WHERE SessionString = '(26 char session id)'
...
etc etc, 16 times altogether in one SHOW FULL PROCESSLIST.
Here's the explain:
+----+-------------+----------+-------+---------------+---------------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------+-------+---------------+---------------+---------+-------+------+-------+
| 1 | SIMPLE | Sessions | const | SessionString | SessionString | 96 | const | 1 | |
+----+-------------+----------+-------+---------------+---------------+---------+-------+------+-------+
1 row in set (0.06 sec)