有没有一种方法可以让SQL在查询中更快?

I want to get the number of unique mobile phone entries per day that have been logged to a database and have never appeared in the log. I thought it was a trivial query but shock when the query took 10 minutes on a table with about 900K entries. A sample Select is getting the number of unique mobile phones that were logged on the 9th of April 2015 and had never been logged before. Its like getting who are the truly new visitors to you site on a specific day. I want to get the number of unique mobile phone