[bbk2516] 第52集 Chapter 13Using Oracle Blokcs Efficeintly[01]

Locally Managed Extents

• Create a locally managed tablespace:(以后所有的表空间管理方式都需要使用Locally managed method)

SQL>CREATE TABLESPACE user_data_1
DATAFILE '/u01/oradata/sid_name/1m_1.dbf'
SIZE 100M
EXTENT MANAGEMENT LOCAL
UNIFORM SIZE 2M;

• With the Oracle database the default extent management is local.

Pros(优点) and Cons(缺点) of Large Extents

• Pros
  • -Are less likely to extent dynamically
  • -Deliver small performance benefit
  • -Enable you to read the entire extent map with a single I/O operation
• Cons
  • -Free space may not be availabe
  • -Unused space

The High-Water Mark

图1

图2

高水位计算公式，图解:

The High-Water Mark

• The high-water mark is :
• -Recorded in the segment header block.
• -Set to the begining of the segment on creation
• -Increamented in five-block increments as rows are inserted
• -Reset by the TRUNCATE command
• Never reset by using DELETE statements.

可以从高水位的角度去解释TRUNCATE 与 DELETE的区别.原理上解释.

Table Statistics

Populate the table statistics using the dbms_stats package and then query the values in dba_tables:

SQL> EXECUTE dbms_stats.gather_table_stats('HR','EMPLOYEES');

PL/SQL procedure successfully completed.

SQL> SELECT num_rows,blocks,empty_blocks as empty,avg_space,chain_cnt,avg_row_len
  2  FROM dba_tables
  3  WHERE owner = 'HR'
  4  AND table_name = 'EMPLOYEES';

  NUM_ROWS     BLOCKS      EMPTY  AVG_SPACE  CHAIN_CNT AVG_ROW_LEN
---------- ---------- ---------- ---------- ---------- -----------
       107          5          0          0          0          69

执行完成analyze table employees compute statistics之后，再查询employees表的统计信息statistics;

SQL> SELECT num_rows,blocks,empty_blocks as empty,avg_space,chain_cnt,avg_row_len
  2  FROM dba_tables
  3  WHERE owner = 'HR'
  4  AND table_name = 'EMPLOYEES';

  NUM_ROWS     BLOCKS      EMPTY  AVG_SPACE  CHAIN_CNT AVG_ROW_LEN
---------- ---------- ---------- ---------- ---------- -----------
       107          5          3       6505          0          71

Recovering Space

• Below the high-water mark:
  • Use the Export and Import utilities to:(方法1-1)
    • -Export the table
    • -Drop or truncate the table
    • -Import the table
  • Or use the Alter Table Employees Move command to move the table.（方法1-2）
• Above the high-water mark,use the Alter Table Employees Deallocate Unused;command.

Example

关于"Above the high-water mark,use the Alter Table Employees Deallocate Unused;command."应用举例

实验背景:

用户MARK下有新创建一张表表T,表中存放了1000000条记录，总共占用了23个extent（在创建表空间时，分配的extent空间大小为1M/个）;

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

     BYTES     BLOCKS    EXTENTS
---------- ---------- ----------
  24117248       2944         23

SQL> anALTER TABLE T ALLOCATE EXTENT;

Table altered.

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

     BYTES     BLOCKS    EXTENTS
---------- ---------- ----------
  25165824       3072         24

现在将多余的那个extent给recovering space.

SQL> ALTER TABLE T DEALLOCATE UNUSED;

Table altered.

SQL> analyze table t compute statistics;

Table analyzed.

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

     BYTES     BLOCKS    EXTENTS
---------- ---------- ----------
  24117248       2944         23