高水位线（HWL）下的许多数据块都是无数据的，但全表扫描的时候要扫描到高水位线的数据块，也就是说oracle要做许多的无用功！因此oracle提供了shrink space碎片整理功能。对于索引，可以采取rebuild online的方式进行碎片整理，一般来说，经常进行DML操作的对象DBA要定期进行维护，同时注意要及时更新统计信息！
一：准备测试数据，使用HR用户，创建T1表，插入约30W的数据，并根据object_id创建普通索引，表占存储空间34M
SQL> conn /as sysdba
已连接。
SQL> select default_tablespace from dba_users where username='HR';
DEFAULT_TABLESPACE
------------------------------------------------------------
USERS
SQL> conn hr/hr
已连接。
SQL> insert into t1 select * from t1;
已创建 74812 行。
SQL> insert into t1 select * from t1;
已创建 149624 行。
SQL> commit;
提交完成。
SQL> create index idx_t1_id on t1(object_id);
索引已创建。
SQL> exec dbms_stats.gather_table_stats('HR','T1',CASCADE=>TRUE);
PL/SQL 过程已成功完成。
SQL> select count(1) from t1;
COUNT(1)
----------
299248
SQL> select sum(bytes)/1024/1024 from dba_segments where segment_name='T1';
SUM(BYTES)/1024/1024
--------------------
34.0625
SQL> select sum(bytes)/1024/1024 from dba_segments where segment_name='IDX_T1_ID';
SUM(BYTES)/1024/1024
--------------------
6
二：估算表在高水位线下还有多少空间可用，这个值应当越低越好，表使用率越接近高水位线，全表扫描所做的无用功也就越少!
DBMS_STATS包无法获取EMPTY_BLOCKS统计信息，所以需要用analyze命令再收集一次统计信息
SQL> SELECT blocks, empty_blocks, num_rows FROM user_tables WHERE table_name ='T1';
BLOCKS EMPTY_BLOCKS NUM_ROWS
---------- ------------ ----------
4302 0 299248
SQL> analyze table t1 compute statistics;
表已分析。
SQL> SELECT blocks, empty_blocks, num_rows FROM user_tables WHERE table_name ='T1';
BLOCKS EMPTY_BLOCKS NUM_ROWS
---------- ------------ ----------
4302 50 299248
SQL> col table_name for a20
SQL> SELECT TABLE_NAME,
2 (BLOCKS * 8192 / 1024 / 1024) -
3 (NUM_ROWS * AVG_ROW_LEN / 1024 / 1024) "Data lower than HWM in MB"
4 FROM USER_TABLES
5 WHERE table_name = 'T1';
TABLE_NAME Data lower than HWM in MB
-------------------- -------------------------
T1 5.07086182
三: 查看执行计划，全表扫描大概需要消耗CPU 1175
SQL> explain plan for select * from t1;
已解释。
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 299K| 28M| 1175 (1)| 00:00:15 |
| 1 | TABLE ACCESS FULL| T1 | 299K| 28M| 1175 (1)| 00:00:15 |
--------------------------------------------------------------------------
四：删除大部分数据，收集统计信息，全表扫描依然需要消耗CPU 1168
SQL> delete from t1 where object_id>100;
已删除298852行。
SQL> commit;
提交完成。
SQL> select count(*) from t1;
COUNT(*)
----------
396
SQL> exec dbms_stats.gather_table_stats('HR','T1',CASCADE=>TRUE);
PL/SQL 过程已成功完成。
SQL> analyze table t1 compute statistics;
表已分析。
SQL> SELECT blocks, empty_blocks, num_rows FROM user_tables WHERE table_name ='T1';
BLOCKS EMPTY_BLOCKS NUM_ROWS
---------- ------------ ----------
4302 50 396
SQL> explain plan for select * from t1;
已解释。
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 396 | 29700 | 1168 (1)| 00:00:15 |
| 1 | TABLE ACCESS FULL| T1 | 396 | 29700 | 1168 (1)| 00:00:15 |
--------------------------------------------------------------------------
五：估算表在高水位线下还有多少空间是无数据的，但在全表扫描时又需要做无用功的数据
SQL> SELECT TABLE_NAME,
2 (BLOCKS * 8192 / 1024 / 1024) -
3 (NUM_ROWS * AVG_ROW_LEN / 1024 / 1024) "Data lower than HWM in MB"
4 FROM USER_TABLES
5 WHERE table_name = 'T1';
TABLE_NAME Data lower than HWM in MB
-------------------- -------------------------
T1 33.5791626
六：对表进行碎片整理，重新收集统计信息
SQL> alter table t1 enable row movement;
表已更改。
SQL> alter table t1 shrink space cascade;
表已更改。
SQL> select sum(bytes)/1024/1024 from dba_segments where segment_name='T1';
SUM(BYTES)/1024/1024
--------------------
.125
SQL> select sum(bytes)/1024/1024 from dba_segments where segment_name='IDX_T1_ID
';
SUM(BYTES)/1024/1024
--------------------
.0625
SQL> SELECT TABLE_NAME,
2 (BLOCKS * 8192 / 1024 / 1024) -
3 (NUM_ROWS * AVG_ROW_LEN / 1024 / 1024) "Data lower than HWM in MB"
4 FROM USER_TABLES
5 WHERE table_name = 'T1';
TABLE_NAME Data lower than HWM in MB
-------------------- -------------------------
T1 33.5791626
SQL> exec dbms_stats.gather_table_stats('HR','T1',CASCADE=>TRUE);
PL/SQL 过程已成功完成。
这个时候，只剩下0.1M的无用功了，执行计划中，全表扫描也只需要消耗CPU 3
SQL> SELECT TABLE_NAME,
2 (BLOCKS * 8192 / 1024 / 1024) -
3 (NUM_ROWS * AVG_ROW_LEN / 1024 / 1024) "Data lower than HWM in MB"
4 FROM USER_TABLES
5 WHERE table_name = 'T1';
TABLE_NAME Data lower than HWM in MB
-------------------- -------------------------
T1 .010738373
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 3617692013
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 396 | 29700 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| T1 | 396 | 29700 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
总共只有5个块，空块却有50个，明显empty_blocks信息过期
SQL> select blocks,empty_blocks,num_rows from user_tables where table_name='T1';
BLOCKS EMPTY_BLOCKS NUM_ROWS
---------- ------------ ----------
5 50 396
SQL> analyze table t1 compute statistics;
表已分析。
SQL> select blocks,empty_blocks,num_rows from user_tables where table_name='T1';
BLOCKS EMPTY_BLOCKS NUM_ROWS
---------- ------------ ----------
5 3 396
