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  • Chapter 11Creating 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.

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  • 原文地址:https://www.cnblogs.com/arcer/p/3015157.html
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