Oracle表碎片整理操作步骤详解

小蚂蚁

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