Chapter 11-Creating Other Schema Objects ---Index

Indexes是可选项，可有可无；它存在的价值就是improves the performance of some queries.

An index:

• Is a schema object Can be used by the Oracle server to speed up the retrieval of rows by using a pointer
• Can reduce disk input/output(I/O) by using a rapid path access method to locate data quickly
• Is independent of the table that it indexes
• Is used and maintained automatically by the Oracle server.

How are indexes created ?

two methods:  

　　|-Auto  

　　|-Manu Automatically:

A unique index is created automatically when you define a PRIMARY KEY or UNIQUE constraint in a table definition.

Manually:Users can create nonunique indexes on columns to speed up access to the rows.

Index category  

　　|-unique index  

　　|-nonunique index  

Creating an index

Create an index on one or more columns:

CREATE [UNIQUE] [BITMAP] INDEX index ON table (column,[,column]...);

Improve the speed of query access to the LAST_NAME column in the EMPLOYEES table: 

CREATE INDEX emp_last_name_idx ON employees(last_name);

Index creation guidelines

Create an index when:

• A column contains a wide range of values
• A column contains a large number of null values One or more columns are frequently used togerther in a WHERE cluase or join condition
• The table is large and most queries are expected to retrieve less than 2% to 4% the rows in the table

Do not create an index when:

• The column are not often used as a condition in the query
• The table is small or most queries are expected to retrieve more than 2% to 4% of the rows in the table
• The table is updated frequently
• The indexed columns are referenced as part of an expression.

Removing an index

Remove an index from the data dictionary by using the DROP INDEX command: DROP INDEX index

Remove the emp_last_name_idx index from the data dictionary DROP INDEX EMP_LAST_NAME_IDX;

To drop an index,you must be the owner of the index or have the DROP ANY INDEX privilege.