Not use foreign keys.

Question

I would like to know if is it bad if I don't want to use foreign keys? is it just a constraint? Is it useful? If I wont use delete on cascade for example is it mandatory to use foreign keys? Please help me with this doubt.
Thanks.

Answer 1

A Small example:
DROP TABLE T_TABLES ;
CREATE TABLE T_TABLES AS
SELECT ROWNUM AS ID, OWNER, TABLE_NAME
FROM   ALL_TABLES;
ALTER TABLE T_TABLES ADD CONSTRAINTS PK_TABLES PRIMARY KEY(ID);
DROP TABLE T_TABLE_COLUMNS ;
CREATE TABLE T_TABLE_COLUMNS AS
  SELECT T.ID, TC.OWNER, TC.TABLE_NAME, TC.COLUMN_NAME, TC.DATA_TYPE
  FROM   ALL_TAB_COLS TC
           JOIN T_TABLES T ON (T.OWNER = TC.OWNER AND T.TABLE_NAME = TC.TABLE_NAME)
  WHERE  T.TABLE_NAME NOT IN ('T_TABLES', 'T_TABLE_COLUMNS'); -- EXCEPT THESE TWO TABLES
EXPLAIN PLAN FOR
SELECT *
FROM   T_TABLE_COLUMNS
WHERE  ID IN (SELECT ID FROM T_TABLES);
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);
| Id  | Operation          | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |                 | 27823 |  3586K|    63   (7)| 00:00:01 |
|   1 |  NESTED LOOPS      |                 | 27823 |  3586K|    63   (7)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| T_TABLE_COLUMNS | 27823 |  3233K|    60   (2)| 00:00:01 |
|*  3 |   INDEX UNIQUE SCAN| PK_TABLES       |     1 |    13 |     0   (0)| 00:00:01 |
--ADD FOREIGN KEY
ALTER TABLE T_TABLE_COLUMNS ADD CONSTRAINT FK_TABLES FOREIGN KEY(ID) REFERENCES T_TABLES(ID);
-- SAME QUERY AGAIN
EXPLAIN PLAN FOR
SELECT *
FROM   T_TABLE_COLUMNS
WHERE  ID IN (SELECT ID FROM T_TABLES);
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);
-- NO JOIN ANYMORE
| Id  | Operation         | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |                 | 27823 |  3233K|    60   (2)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T_TABLE_COLUMNS | 27823 |  3233K|    60   (2)| 00:00:01 |
-------------------------------------------------------------------------------------FKs can even save you from joins.