I think that the multi table approach is a bad one. MySQL's single unique key lookups are really fast, and you can create a single ID by concatenating the keys in sorted order with their values.

While 10 tables (and thus 30 files in MySQL) is not a lot, it is better to avoid multiple tables.

Also you have the problem of cache expiry - because you should go through and clear out the expired data. This adds a level of complication where you first have to figure out what tables you have then expire the data in all of those.

MySQL works very well with large tables - just do an EXPLAIN SELECT with a single unique key and you will see how little work it has to do to return your required results.