Query performance

Hi all,

Hope every one is fine.

I have a query which look like this, i have a performance problem with this query

select a.sex,a.age_group,b.organisation,b.name ,sum(b.popualtion),sum(b.population)

From table1 a inner join lookup_table b

on a.age_group_key=b.age_group_key

and a.sex_key=b.sex_key

group by 

a.sex,a.age_group,b.organisation,b.name

I am calculating population in region standardized by age and sex  in the above query. the main table table1 got 1.5 millions rows and lookup table got around 1000 values in it.

Instead of keys if i join on normal age group and sex columns the query takes same amount of time don't know why,

can anyone explain me why please.

thanks

RRavilla