关于Oracle表碎片整理
数据库在日常使用过程中，不断的insert，delete，update操作，导致表和索引出现碎片是在所难免的事情，碎片多了，sql的执行效率自然就差了，道理很简单，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左右，索引占6M左右的存储空间[*]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
参考：http://surachartopun.com/2011/08/determine-hwm-and-reduce-it-by-shrink.html
关于如何确定哪些表需要进行碎片整理，可以使用附件中的脚本去查询，具体请参考：http://www.toadworld.com/KNOWLEDGE/KnowledgeXpertforOracle/tabid/648/TopicID/OTNN18/Default.aspx
页:
[1]