Thanks, but my question is more about how to perform the search across the different tables (I know I must add indexes).
More explicitely, I want to find the restaurants having:
-all or some of the search strings in any text fields (text, varchar...) of the 'restaurants' table
-'city_id' and/or 'country_id' corresponding to cities or countries name matching some of the search strings
-their ids in rows of the 'comments' table which 'comment' field contain some of the search strings.