Question

I am using mySQL 5.1 and am wondering, is it necessary to add CREATE INDEX syntax on a column after you have defined it in a FK relationship. Common sense tells me that if a column is a key (foreign or otherwise), then it needs to be indexed – but you never know …

Do I have to EXPLICITLY create an index on my FK – or does creating an FK implicitly creates an index on the column ?

Answer 1

Quoting this page of the MySQL manual : 13.6.4.4. FOREIGN KEY Constraints :

InnoDB requires indexes on foreign
keys and referenced keys so that
foreign key checks can be fast and not
require a table scan.
In the
referencing table, there must be an
index where the foreign key columns
are listed as the first columns in the
same order.
Such an index is
created on the referencing table
automatically if it does not exist.