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oracle表碎片的整理分析

发表于：2024-11-23 作者：千家信息网编辑

千家信息网最后更新 2024年11月23日，本篇文章给大家分享的是有关oracle表碎片的整理分析，小编觉得挺实用的，因此分享给大家学习，希望大家阅读完这篇文章后可以有所收获，话不多说，跟着小编一起来看看吧。数据库在日常使用过程中，不断的ins

千家信息网最后更新 2024年11月23日oracle表碎片的整理分析

本篇文章给大家分享的是有关oracle表碎片的整理分析，小编觉得挺实用的，因此分享给大家学习，希望大家阅读完这篇文章后可以有所收获，话不多说，跟着小编一起来看看吧。

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!

表的碎片和文件系统的碎片化的是不一样的,当随着在一个表上的DML的上操作越来越多时,HWM之前可能有很多空闲空间,而在读取表时HWM以下的块都会被读进来,这样会产生更多的IO,从而影响性能.只有在DDL操作才会进表的收缩.

对表进行碎片整理，碎片整理方法有:1,使用alter MOVE 表，然后索引rebuild;2.使用alter table enable row movement;然后alter table shrink space cascade(shrink使用有限制，需注意)；3,通过 create table XXX as select * from abb; 4,使用导出和导入表 ;

实验如下：
SQL> create table t1 as select * from dba_objects;

Table created.

SQL> select count(*) from t1;

COUNT(*)
----------
86956

SQL> insert into t1 select * from t1;

86956 rows created.

SQL> commit;

Commit complete.

SQL>
SQL> create index idx_t1_id on t1(object_id);

Index created.

--先查询表大小及统计信息：
SQL> set lines 200
SQL> COL TABLE_NAME FOR A15
SQL> COL TABLESPACE_NAME FOR A15
SQL> select OWNER,TABLE_NAME,TABLESPACE_NAME,NUM_ROWS,BLOCKS,EMPTY_BLOCKS,AVG_SPACE,AVG_ROW_LEN,LAST_ANALYZED from dba_tables where TABLE_NAME='T1';
OWNER TABLE_NAME TABLESPACE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS AVG_SPACE AVG_ROW_LEN LAST_ANALYZED
------------------------------ --------------- --------------- ---------- ---------- ------------ ---------- ----------- -------------------
SYS T1 SYSTEM

SQL> COL SEGMENT_NAME FOR A15
SQL> select OWNER,SEGMENT_NAME,TABLESPACE_NAME,BYTES,BLOCKS,EXTENTS from dba_segments where segment_name in ('T1','IDX_T1_ID');

OWNER SEGMENT_NAME TABLESPACE_NAME BYTES BLOCKS EXTENTS
------------------------------ --------------- ------------------------------ ---------- ---------- ----------
SYS T1 SYSTEM 20971520 2560 35
SYS IDX_T1_ID SYSTEM 4194304 512 19

可以看到dba_segment中已经可以记录表大小，而dba_tables则没有。

--使用dbms_stats手机统计信息
SQL> exec dbms_stats.gather_table_stats('SYS','T1',CASCADE=>TRUE);

PL/SQL procedure successfully completed.

SQL>

SQL> select OWNER,TABLE_NAME,TABLESPACE_NAME,NUM_ROWS,BLOCKS,EMPTY_BLOCKS,AVG_SPACE,AVG_ROW_LEN,LAST_ANALYZED from dba_tables where TABLE_NAME='T1';

OWNER TABLE_NAME TABLESPACE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS AVG_SPACE AVG_ROW_LEN LAST_ANALYZED
------------------------------ --------------- --------------- ---------- ---------- ------------ ---------- ----------- -------------------
SYS T1 SYSTEM 173912 2476 0 0 98 2017-10-26 05:35:37

SQL> select OWNER,SEGMENT_NAME,TABLESPACE_NAME,BYTES,BLOCKS,EXTENTS from dba_segments where segment_name in ('T1','IDX_T1_ID');

OWNER SEGMENT_NAME TABLESPACE_NAME BYTES BLOCKS EXTENTS
------------------------------ --------------- --------------- ---------- ---------- ----------
SYS T1 SYSTEM 20971520 2560 35
SYS IDX_T1_ID SYSTEM 4194304 512 19

发现dba_table中已有记录BLOCKS块大小记录，但是没有empty_blocks空块记录和AVG_SPACE值。

--需要使用analyze子句收集表t1的空块信息。
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> analyze table t1 compute statistics;

Table analyzed.

SQL> select OWNER,TABLE_NAME,TABLESPACE_NAME,NUM_ROWS,BLOCKS,EMPTY_BLOCKS,AVG_SPACE,AVG_ROW_LEN,LAST_ANALYZED from dba_tables where TABLE_NAME='T1';

OWNER TABLE_NAME TABLESPACE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS AVG_SPACE AVG_ROW_LEN LAST_ANALYZED
------------------------------ --------------- --------------- ---------- ---------- ------------ ---------- ----------- -------------------
SYS T1 SYSTEM 173912 2476 83 863 101 2017-10-26 05:38:18

SQL> select OWNER,SEGMENT_NAME,TABLESPACE_NAME,BYTES,BLOCKS,EXTENTS from dba_segments where segment_name in ('T1','IDX_T1_ID');

OWNER SEGMENT_NAME TABLESPACE_NAME BYTES BLOCKS EXTENTS
------------------------------ --------------- --------------- ---------- ---------- ----------
SYS T1 SYSTEM 20971520 2560 35
SYS IDX_T1_ID SYSTEM 4194304 512 19

发现dba_tables中的 EMPTY_BLOCKS AVG_SPACE AVG_ROW_LEN字段有值了，且AVG_ROW_LEN的值发生了变化。

--计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，如下：
SQL> select table_name, (blocks * 8192 / 1024 / 1024) - (num_rows * avg_row_len / 1024 / 1024) "data lower than hwm in mb" from user_tables where table_name = 'T1';

TABLE_NAME data lower than hwm in mb
--------------- -------------------------
T1 2.59235382

查看执行计划，全表扫描大概需要消耗CPU 675
SQL> explain plan for select * from t1;

Explained.

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 | | 173K| 16M| 675 (1)| 00:00:09 |
| 1 | TABLE ACCESS FULL| T1 | 173K| 16M| 675 (1)| 00:00:09 |
--------------------------------------------------------------------------

8 rows selected.

--删除大部分数据，收集统计信息，全表扫描依然耗cpu 673，如下：
SQL> select count(*) from t1;

COUNT(*)
----------
173912

SQL> delete t1 where rownum <170000;

169999 rows deleted.

SQL> select count(*) from t1;

COUNT(*)
----------
3913

--使用dbms_stats分析表
SQL> exec dbms_stats.gather_table_stats('SYS','T1',CASCADE=>TRUE);

PL/SQL procedure successfully completed.

--再次查询dba_segments和dba_tables视图
SQL> select OWNER,TABLE_NAME,TABLESPACE_NAME,NUM_ROWS,BLOCKS,EMPTY_BLOCKS,AVG_SPACE,AVG_ROW_LEN,LAST_ANALYZED from dba_tables where TABLE_NAME='T1';

OWNER TABLE_NAME TABLESPACE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS AVG_SPACE AVG_ROW_LEN LAST_ANALYZED
------------------------------ --------------- --------------- ---------- ---------- ------------ ---------- ----------- -------------------
SYS T1 SYSTEM 3913 2476 83 863 101 2017-10-26 05:50:29

SQL> select OWNER,SEGMENT_NAME,TABLESPACE_NAME,BYTES,BLOCKS,EXTENTS from dba_segments where segment_name in ('T1','IDX_T1_ID');

OWNER SEGMENT_NAME TABLESPACE_NAME BYTES BLOCKS EXTENTS
------------------------------ --------------- --------------- ---------- ---------- ----------
SYS T1 SYSTEM 20971520 2560 35
SYS IDX_T1_ID SYSTEM 4194304 512 19

发现dba_tables中的num_rows字段已经更新了，其他字段没有更新；而dba_segments视图相关字段也没有变化。这说明DML操作的删除行操作，即使进行了统计信息的更新，但是因为表里存在碎片，所以表大小没有变化。

--使用analyze分析表：
SQL> analyze table t1 compute statistics;

Table analyzed.

SQL> select OWNER,TABLE_NAME,TABLESPACE_NAME,NUM_ROWS,BLOCKS,EMPTY_BLOCKS,AVG_SPACE,AVG_ROW_LEN,LAST_ANALYZED from dba_tables where TABLE_NAME='T1';

OWNER TABLE_NAME TABLESPACE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS AVG_SPACE AVG_ROW_LEN LAST_ANALYZED
------------------------------ --------------- --------------- ---------- ---------- ------------ ---------- ----------- -------------------
SYS T1 SYSTEM 3913 2476 83 7761 104 2017-10-26 05:52:00

SQL> select OWNER,SEGMENT_NAME,TABLESPACE_NAME,BYTES,BLOCKS,EXTENTS from dba_segments where segment_name in ('T1','IDX_T1_ID');

OWNER SEGMENT_NAME TABLESPACE_NAME BYTES BLOCKS EXTENTS
------------------------------ --------------- --------------- ---------- ---------- ----------
SYS T1 SYSTEM 20971520 2560 35
SYS IDX_T1_ID SYSTEM 4194304 512 19

还是没有变化，结论如上。。。。。。。。。。。。。。。。。

--查看执行计划，cpu cost 673几乎没变化
SQL> explain plan for select * from t1;

Explained.

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 | | 3913 | 397K| 673 (1)| 00:00:09 |
| 1 | TABLE ACCESS FULL| T1 | 3913 | 397K| 673 (1)| 00:00:09 |
--------------------------------------------------------------------------

8 rows selected.

--再次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块，如下：
SQL> select table_name, (blocks * 8192 / 1024 / 1024) - (num_rows * avg_row_len / 1024 / 1024) "data lower than hwm in mb" from user_tables where table_name = 'T1';

TABLE_NAME data lower than hwm in mb
--------------- -------------------------
T1 18.9556503

发现表中碎片增长很多。。。。。。。。。。。。

--对表进行碎片整理，重新收集统计信息，如下：
注：碎片整理方法有:1,使用alter MOVE 表，然后索引rebuild;2.使用alter table enable row movement;然后alter table shrink space cascade(shrink使用有限制，需注意)；3,通过 create table XXX as select * from abb; 4,使用导出和导入表 ;

SQL> alter table t1 disable row movement;

Table altered.

SQL> alter table t1 move;

Table altered.

SQL> select INDEX_NAME,STATUS from dba_indexes where index_name ='IDX_T1_ID';

INDEX_NAME STATUS
------------------------------ --------
IDX_T1_ID UNUSABLE

SQL> alter index IDX_T1_ID rebuild online;

Index altered.

--先查询dba_tables/dba_segments：
SQL> select OWNER,TABLE_NAME,TABLESPACE_NAME,NUM_ROWS,BLOCKS,EMPTY_BLOCKS,AVG_SPACE,AVG_ROW_LEN,LAST_ANALYZED from dba_tables where TABLE_NAME='T1';

OWNER TABLE_NAME TABLESPACE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS AVG_SPACE AVG_ROW_LEN LAST_ANALYZED
------------------------------ --------------- --------------- ---------- ---------- ------------ ---------- ----------- -------------------
SYS T1 SYSTEM 3913 2476 83 7761 104 2017-10-26 05:52:00

SQL> select OWNER,SEGMENT_NAME,TABLESPACE_NAME,BYTES,BLOCKS,EXTENTS from dba_segments where segment_name in ('T1','IDX_T1_ID');

OWNER SEGMENT_NAME TABLESPACE_NAME BYTES BLOCKS EXTENTS
------------------------------ --------------- --------------- ---------- ---------- ----------
SYS T1 SYSTEM 524288 64 8
SYS IDX_T1_ID SYSTEM 131072 16 2

发现经过碎片整理后且在没有收集统计信息的情况下dba_segments的块大小已经自动更新了，而dba_tables各字段没有更新

--再次查询碎片情况：
SQL> select table_name, (blocks * 8192 / 1024 / 1024) - (num_rows * avg_row_len / 1024 / 1024) "data lower than hwm in mb" from user_tables where table_name = 'T1';

TABLE_NAME data lower than hwm in mb
--------------- -------------------------
T1 18.9556503

没有变化。

--收集统计信息，使用dbms_stat包：
SQL> exec dbms_stats.gather_table_stats('SYS','T1',CASCADE=>TRUE);

PL/SQL procedure successfully completed.

SQL>
SQL> select OWNER,TABLE_NAME,TABLESPACE_NAME,NUM_ROWS,BLOCKS,EMPTY_BLOCKS,AVG_SPACE,AVG_ROW_LEN,LAST_ANALYZED from dba_tables where TABLE_NAME='T1';

OWNER TABLE_NAME TABLESPACE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS AVG_SPACE AVG_ROW_LEN LAST_ANALYZED
------------------------------ --------------- --------------- ---------- ---------- ------------ ---------- ----------- -------------------
SYS T1 SYSTEM 3913 58 83 7761 101 2017-10-26 06:07:17

SQL> select OWNER,SEGMENT_NAME,TABLESPACE_NAME,BYTES,BLOCKS,EXTENTS from dba_segments where segment_name in ('T1','IDX_T1_ID');

OWNER SEGMENT_NAME TABLESPACE_NAME BYTES BLOCKS EXTENTS
------------------------------ --------------- --------------- ---------- ---------- ----------
SYS T1 SYSTEM 524288 64 8
SYS IDX_T1_ID SYSTEM 131072 16 2

SQL> select table_name, (blocks * 8192 / 1024 / 1024) - (num_rows * avg_row_len / 1024 / 1024) "data lower than hwm in mb" from user_tables where table_name = 'T1';

TABLE_NAME data lower than hwm in mb
--------------- -------------------------
T1 .076220512

发现经过dbms_stat包收集统计信息后dba_tables的blocks、AVG_ROW_LEN字段已经更新，且高水位下的碎片已经回收了，但是EMPTY_BLOCKS、AVG_SPACE字段没有更新

--使用analyze子句收集EMPTY_BLOCKS字段统计信息，如下；
SQL> analyze table t1 compute statistics;

Table analyzed.

SQL> select OWNER,TABLE_NAME,TABLESPACE_NAME,NUM_ROWS,BLOCKS,EMPTY_BLOCKS,AVG_SPACE,AVG_ROW_LEN,LAST_ANALYZED from dba_tables where TABLE_NAME='T1';

OWNER TABLE_NAME TABLESPACE_NAME NUM_ROWS BLOCKS EMPTY_BLOCKS AVG_SPACE AVG_ROW_LEN LAST_ANALYZED
------------------------------ --------------- --------------- ---------- ---------- ------------ ---------- ----------- -------------------
SYS T1 SYSTEM 3913 58 5 887 104 2017-10-26 06:10:06

SQL> select OWNER,SEGMENT_NAME,TABLESPACE_NAME,BYTES,BLOCKS,EXTENTS from dba_segments where segment_name in ('T1','IDX_T1_ID');

OWNER SEGMENT_NAME TABLESPACE_NAME BYTES BLOCKS EXTENTS
------------------------------ --------------- --------------- ---------- ---------- ----------
SYS T1 SYSTEM 524288 64 8
SYS IDX_T1_ID SYSTEM 131072 16 2

SQL> select table_name, (blocks * 8192 / 1024 / 1024) - (num_rows * avg_row_len / 1024 / 1024) "data lower than hwm in mb" from user_tables where table_name = 'T1';

TABLE_NAME data lower than hwm in mb
--------------- -------------------------
T1 .06502533

发现经过analyze子句收集统计信息后dba_tables的EMPTY_BLOCKS、AVG_SPACE字段更新了

--再次执行sql，发现CPU cost只有17，如下：
SQL> explain plan for select * from t1;

Explained.

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 | | 3913 | 397K| 17 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| T1 | 3913 | 397K| 17 (0)| 00:00:01 |
--------------------------------------------------------------------------

8 rows selected.