[bbk4998] 第104集 第13章 表空间管理 02

BitMap信息是保存在段头信息.

录入信息的时候，就是直接读取段头信息，不在读取数据字典中的信息；

以前使用数据字典方式进行管理的时候，当多用户多并发的操作发生时，就会发生高负荷的读写系统表空间的操作,增加系统负荷.因为数据字典的信息是存放在系统表空间当中的.

段就是存储各种类型数据的对象，段的空间分配是通过extent来实现的.

oracle采用延迟创建段技术，提高效率；当一个表创建的时候，不给它创建对应的段空间信息，只有在使用的时候才创建；

没有段信息的索引或者分区索引是不可用的.

在10g以前的版本中，将索引置为不可用的时候，其所占用的空间依然是存在的；在10g，11g之后，删除了或者设为不可用之后，其所占的空间会释放.

ALTER INDEX test_i UNUSABLE;设置索引不可用，同时释放所占资源空间.如果想恢复索引，则直接rebuild即可.

Free Space Management Within Segments

• Tracked by bitmaps in segments
• Benefits:
• More flexible space utilization
• Run-time adjustment
• Multiple process search of BMBs

Types of Segments

A segment is a set of extents allocated for a certain logical structure.The different types of segments include:

• Table and cluster segments
• Index segment
• Undo segment
• Temporary segment

Segments are dynamically allocated by the Oracle database server.

Segments的四大类型

Allocating Extents

• Searching the data file`s bitmap for the required number of adjacent free blocks
• Sizing extents with storage clauses:
  • -UNIFORM
  • -AUTOALLOCATE
• View extent map
• Obtaining deallocation advice
  • 通过查看extent map之后，可以适当进行deallocation advice操作

Allocating Space

New space allocation method:

SQL> show parameter deferr

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
deferred_segment_creation            boolean     TRUE

1. Table creation > Data dictionary operation
2. DML > Segment creation

使用延迟创建segment信息的益处:

• Saving disk space
• Improving installation time of big applications

Creating Tables without segments

SQL> show parameter deferr

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
deferred_segment_creation            boolean     TRUE
SQL> conn test/test
Connected.

SQL> create table seg_test(c number,d varchar2(500));

Table created.

SQL> select segment_name from user_segments;

SEGMENT_NAME
--------------------------------------------------------------------------------
EMP1
EMP_EMPLOYEE_ID_IDX

SQL> insert into seg_test values(1,'abcdefg');

1 row created.

SQL> select segment_name from user_segments;

SEGMENT_NAME
--------------------------------------------------------------------------------
EMP1
SEG_TEST
EMP_EMPLOYEE_ID_IDX

上面示例说明，在创建新表的时候，未插入新的数据之前，是没有分配段信息的.

Controlling Deferred Segment Creation

With the DEFERRED_SEGMENT_CREATION parameter in the:

• Initialization file
• ALTER SESSION command
• ALTER SYSTEM command

With the SEGMENT CREATION clause

• IMMEDIATE
• DEFERRED(default in Oracle Database 11gR2)

CREATE TABLE SEG_TAB3(C1 number,C2 number) SEGMENT CREATION IMMEDIATE TABLESPACE SEG_TBS;

CREATE TABLE SEG_TAB4(C1 number,C2 number) SEGMENT CREATION DEFERRED;

Note:Indexes inherit table characteristics.

Restrictions and Exceptions

Segment creation on demand:

• Only for nonpartitioned tables and indexes
• Not for IOTs,clustered tables,or other special tables
• Not for tables in dictionary-managed tablespaces

Note:If you were to migrate a table without segments from a locally managed to a dictionary-managed tablespace,you must drop and re-create it.

Additional Automatic Functionality

without user intervention:

• No segments for unusable indexes and index partitions
• Creating an index without a segment

CREATE INDEX test_i1 ON seg_test(c) UNUSABLE;

• Removing any allocated space for an index:

ALTER INDEX test_i UNUSABLE;

• Creating the segment for an index:

ALTER INDEX test_i REBUILD;

SELECT segment_name,partition_name,segment_type

FROM user_segments

WHERE segment_name like '%DEMO%';