select where index + other field Learning indexing

I have made an index, consisting of usergroupid and birthdate

I understand that queries will then benefit from indexing when searching usergroupid and/or birthdate

But what happens if I throw in other fields in the query?

SELECT * FROM mytable WHERE usergroupid = 54 AND (birthdate BETWEEN '1980-01-01' AND '1983-01-01') AND cityid = 100


Will the extra "AND cityid = 100" mess up the indexing, or will it be seen as something to be found quickly AFTER handling the index?
The extra predicate wont mess up the indexing. Most likely your index will be evaluated first and any rows that are found will be evaluated against the predicate cityid = 100. 

On the other hand, assuming your index is defined as (usergroupid, birthdate). If you remove usergroupid = 54 from your query, the index won't do any good and you will have to evaluate all rows in the table against:  birthdate BETWEEN '1980-01-01' AND '1983-01-01') AND cityid = 100