
In Figure 5–30, another index is applied to the role played by Country in the associ-
ation Person was born in Country. If you often want to inquire about where a person was
born, and you want the name of the country, not just the country-code, when you make
such a query, then this entails a frequent conceptual join operation between the two
roles played by Country. These roles map to separate tables, one role mapping to a for-
eign key and the other to its referenced primary key. At the relational level, this means
we have a need to efficiently join the tables by matching the foreign and primary key
values. In this case, you can add an index on the ...