MySQL server taking multiple seconds to run COUNT(*) query using an index on a MyISAM table
The problem appears to have been that the cardinality of my indices was NULL. I am unsure why the indices would revert to this state when they were apparently fine yesterday.

I discovered this problem by running SHOW INDEX FROM tickets.

I ran ANALYZE TABLE tickets. Now the EXPLAIN looks subtly different-- the rows have dropped to 1:

mysql&gt; explain select count(*) from tickets WHERE id = 214;
|  1 | SIMPLE      | tickets | const | PRIMARY       | PRIMARY | 2       | const |    1 | Using index |