Foreign key and index

Question

In our table:
Table Name:
STATUS
Columns:
STATUSID NUMBER (PKEY)
USERID NUMBER (FKEY)
PATIENTNAME VARCHAR(30)
STATUS_DATE DATE
CREATE_DATE DATE
CHSTATUS VARCHAR(30)
PAYER_STATUS VARCHAR(30)
CHARGE_AMOUNT DOUBLE
INS_AMOUNT DOUBLE
X12IN CLOB
We have a foreign key on the USERID column. And this column appears in the WHERE clause always.
I wanted to know in addition to the foriegn key constraint, putting an INDEX on the USERID column will help query performance?
Generally speaking:
I wanted to know whether in oracle putting a foreign key constraint on a column creates an INDEX automatically or do I put the index specifically besides the FOREIGN key.
Thanks in Advance,
Rumpa Giri

Answer 1

Hello
Oracle does NOT create an index when you put a foreign key constraint on a column, so you need to create one if you want one.
Have a look at this discussion on asktom to understand when and why indexes on foreign keys are a good idea.
If your query that uses USERID in the where caluse returns a small number of rows in proportion to the number of rows in the table, adding an index could well help with query performance....but as always, it's best to benchmark it and get some stats that you can then use to make an informed choice.
HTH
David
Message was edited by:
david_tyler