You can add an index to any existing table. You use the same basic syntax to create an index as to create other database objects using Data Definition Language.
The basic syntax for creating an index consists of these clauses:
The CREATE keyword is required as the beginning of this SQL statement.
You can use either of these keywords to make the index either unique or use a bitmapped structure. If you do not specify either, you will create an index that allows duplicate values. You can only use one of these keywords, so you cannot specify a unique, bitmapped index.
The INDEX keyword is required as part of this DDL table.
The index_name must be unique for indexes in a particular schema. You also can specify a schema name as a prefix to the index name if you are creating indexes in a schema other than the one belonging to your current username.
ON table name:
The ON keyword is required, and the table_name may have to be qualified with a schema name.
( col_name1, col_name2, ...):
You can specify up to 32 columns in an Oracle index.
These keywords specify that the index should be sorted in ascending (ASC) or descending (DESC) order. The default order is ascending.
If you wanted to add an index for the BIDDER_CLIENT_ID column in the BID table, you would use the following syntax.
CREATE INDEX bid_client_idx
ON bid (bidder_client_id) ASC;
This index will allow duplicate values, because it is not specified as UNIQUE. Although the index will be sorted in ascending order by default, it is always good practice to specify the sort order.
If you define a UNIQUE or PRIMARY KEY constraint, Oracle will, by default, create the appropriate index to support the constraint.
Overview of Indexed Clusters
An indexed cluster is a table cluster that uses an index to locate data. The cluster index is a B-tree index on the cluster key. A cluster index must be created before any rows can be inserted into clustered tables.
Assume that you create the cluster employees_departments_cluster with the cluster key department_id, as shown in Example 5-4. Because the HASHKEYS clause is not specified, this cluster is an indexed cluster.
Afterward, you create an index named idx_emp_dept_cluster on this cluster key.
CREATE INDEX idx_emp_dept_cluster
ON CLUSTER employees_departments_cluster;
You then create the employees and departments tables in the cluster, specifying the department_id column as the cluster key, as follows (the ellipses mark the place where the column specification goes):
Finally, you add rows to the employees and departments tables. The database physically stores all rows for each department from the employees and departments tables in the same data blocks.
The database stores the rows in a heap and locates them with the index.
Figure 5-6 shows the employees_departments_cluster table cluster, which contains employees and departments. The database stores rows for employees in department 20 together, department 110 together.
If the tables are not clustered, then the database does not ensure that the related rows are stored together.
The next lesson explores storage considerations for index structures.