SQL Monitor plugin by vladimir_kolesnikov analyzes every query to the database.
SQLMon tries to help the developers and analyzes the query itself, reporting everything that needs attention.
Every query passed to WordPress is analyzed (using EXPLAIN) and the results are shown in the theme’s footer or Admin panel’s footer (they are visible to site administrators only — this allows to use SQLMon even on live sites).
One of the key features of SQLMon is that it can run EXPLAIN not only on SELECT queries but also on UPDATE/DELETE and INSERT/REPLACE INTO … AS or CREATE TABLE … AS.
PHP 5 only
Plugin homepage: https://launchpad.net/wp-plugin-sqlmon