I noticed that the Aprilon MineCraftApproval system got slower and slower over time so I whipped up a crude profiling mechanism, extending the mysqli class with microtime() time-tracking code and function call-frequency-tracking code using debug_backtrace().
It also prints out  every MySQL query made, the call-stack which led to this query and sorts everything by how long it took for the each transaction.
Bear in mind that this includes processing overhead by the MySQLi PHP extension, libmysql library on the system and sockets opening/closing, receiving and sending if the database is on a remote server.

You can see example output on the right and you may notice a few warnings regarding “No possible_keys”.
That means the query could not use any indices because the table doesn’t have any matching the WHERE clause (cut off on pic.).

The number of warnings shrunk to zero after a few ALTER TABLE, ADD INDEX(..),
and the time to execute the MySQL queries was reduced from 59ms (wolfram: ~blink of an eye) to 16.17ms (wolfram: ~nerve impulse to travel the length of a human).

Excuse my french, but that’s fuckin’ impressive.

Even if’s ‘just’ a difference of  42.84ms, it still makes an impact on the experience of the whole application.