Since the dosearch.php script runs off of post variables you can't easily add debug=1 to the URL to get the full information on the query. So, try this. Open up dosearch.php and right after the calls to get_input() add this:
$debug=1;
Then run a search using the mod you're trying to create. You'll get a full EXPLAIN on the query which will help diagnose what rows the search is grabbing and if it's using any indexes, etc.
