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.