Creation of Indexes
Automatically: When a primary key or Unique constraint is show in a table definition then a unique index is formed automatically.
Manually: User can form non-unique indexes on columns to speed up access time to rows.
Example: The following commands produce index on employee name and employee name + department number respectively.
CREATE INDEX EMP_ENAME_IDX ON EMP (ENAME);
CREATE INDEX EMP_MULTI_IDX ON EMP (ENAME, DEPTNO);
Finding facts about created indexes: The data dictionary have the name of index, table name and column names. For instance, in Oracle a user-indexes and user-ind-columns see contains the details about user formed indexes.
Remove an index from the data dictionary:
DROP INDEX EMP_ENAME_IDX;
Indexes cannot be modified.