[bbk2515] 第51集 - Chapter 13-Using Oracle Blokcs Efficeintly[00]

Objectives

• After completing this lesson,you should be able to do the following:
• Use automatic segment space management
• Use manual segment space management
• Describe the use of Oracle block parameters
• Recover space from sparsely populated segments
• Describe and detect chaining and migration of Oracle blocks
• Perform index reorganization

Database Storage Hierarchy

Allocation of Extents

To avoid the disadvantages of dynamic extent allocation:

避免动态分配extent的方法有三:

• Create locally managed tablespaces.
• Size the segments appropriately.适当的对segment进行大小规划
• Monitor segments ready to extend.作为DBA来讲，要实时监控segment的使用情况，对于一些需要调整的segment，可以事先运行一些脚本来进行合理的分配.

Segment分配的时候，是按照extent来进行的，而不是按照block进行的.

Avoiding Dynamic Allocation

• To display segments with less than 10% free blocks
  • blocks->已经使用过的blocks
  • empty_blocks->从未使用过的blocks
  • 第一次查询时，empty_blocks有可能为NULL或者为0，原因是尚未进行analyze table t compute statistics.命令.

SELECT owner,table_name,blocks,empty_blocks

FROM dba_tables

WHERE empty_blocks / (blocks + empty_blocks) < .1;

 

• To avoid dynamic allocaiton:

SQL>ALTER TABLE HR.employees ALLOCATE EXTENT;

Table altered.

 

SQL> alter table MARK.t ALLOCATE EXTENT;

Table altered.

SQL> alter table MARK.t ALLOCATE EXTENT (size 30M);

Table altered.

 

Example:

CREATE TABLESPACE tbs1 datafile '/u01/oradata/DB202/tbs1_01.dbf' size 200M
EXTENT MANAGEMENT LOCAL UNIFORM SIZE 1M
SEGMENT SPACE MANAGEMENT auto;

SQL> create user mark identified by mark default tablespace tbs1;

User created.

SQL> grant connect,resource to mark;

Grant succeeded.

 

conn as mark

[oracle@DG3 ~]$ sqlplus mark/mark

SQL*Plus: Release 11.2.0.1.0 Production on Wed May 8 11:04:08 2013

Copyright (c) 1982, 2009, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> create table t(id int,name char(10));

Table created.

 

conn as sysdba

SQL> SELECT blocks,empty_blocks FROM dba_tables WHERE owner='MARK' AND table_name='T';

    BLOCKS EMPTY_BLOCKS
---------- ------------

 

conn as mark

SQL> exec dbms_stats.gather_table_stats('MARK','T');

PL/SQL procedure successfully completed.

 

conn as sysdba

SQL> SELECT blocks,empty_blocks FROM dba_tables WHERE owner='MARK' AND table_name='T';

    BLOCKS EMPTY_BLOCKS
---------- ------------
         0            0

 

执行dbms_stats.gather_table_stats后发现，empty_blokcs为0；说明使用此Procedure不能统计出真实的empty_blocks信息，必须使用analyze命令.

conn as mark

SQL> analyze table T compute statistics;

Table analyzed.

 

问题出现了,在9i的环境中，执行完上述SQL CODE之后，查询相应的字典信息，均有正确数据（dba_tables,dba_segments,dba_extents），但是在oracle 11gr2中，查询上述数据字典信息，则没有数据.这是为什么?

SQL> SELECT blocks,empty_blocks FROM dba_tables WHERE owner='MARK' AND table_name='T';

    BLOCKS EMPTY_BLOCKS
---------- ------------
         0            0

SQL> SELECT bytes,blocks,extents from dba_segments where owner='MARK' and segment_name='T';

no rows selected

SQL> SELECT extent_id,block_id,bytes from dba_extents where owner='MARK' and segment_name='T';

no rows selected