Sometimes it's a good idea to take care of your tables. What options are there?
OPTIMIZE TABLE persons
This command is especially useful after you've made a large change to your table. The table as well as the indexes get reorganized and you can free some disk space with it too.
More information here: http://dev.mysql.com/doc/refman/en/optimize-table.html
ANALYZE TABLE persons
This command analyzes the key distribution for the specified table. This helps the query optimizer to do its job better.
More information here: http://dev.mysql.com/doc/refman/en/analyze-table.html
REPAIR TABLE persons
This shouldn't be necessary, but if you have a corrupt table, it might help.
More information here: http://dev.mysql.com/doc/refman/en/repair-table.html