I have a very complicated query which involve more than 10 tables. Some parent tables are loaded only to search on one field. I wonder if I can copy this field in the child table (this data is never updated), so that I don't need to load the parent table in my query.
I've always learned that I should not store the same data in two different place, but maybe it's "allowed" for performance ?
For example, imagine a country table, a city table, and a street table. Having the country code in the street table would speed up some complicated queries.