ORACLE 收缩段（移动HWM）

 

    今天一客户打电话，说对他们的表删除了很多行，但是查看表所站的块没有减少。这是因为HWM没有移动的原因。HWM如最高水位线一样，默认只有增加。

    HWM对数据库的性能影响是很大的：

    1，比如做全表扫描的时候。

    2，在insert语句中增加了append提示的时候。

    降低HWM有很多的方法。

    1，exp/imp表。

    2，create table  as select ....和alter table .. rename ...

    3, move 表。

    4，shrink表（10G）

 

    下面我们介绍move与shrink两种方法。

    move与shrink他们能移动HWM,还能消除部分行迁移，消除空间碎片。

   

    下面是move与shrink的操作。

   

    创建测试数据：

   

SQL> create user luoping identified by htz;

User created.

SQL> grant connect,resource to luoping;

Grant succeeded.

SQL> conn luoping/htz
Connected.

SQL> create table htz1 (a int)
  2  ;

Table created.

SQL> create table htz2 (a int);

Table created.

SQL> create index htz1_a on htz1(a);

Index created.

SQL> create index htz2_a on htz2(a);

Index created.


begin
for i in 1..1000000 loop
insert into htz2 values(i);
insert into htz1 values(i);
if mod(i,1000)=0 then
commit;
end if;
end loop;
end;
/

SQL> begin
for i in 1..1000000 loop
  2    3  insert into htz2 values(i);
insert into htz1 values(i);
  4    5  if mod(i,1000)=0 then
  6  commit;
  7  end if;
  8  end loop;
  9  end;
 10  /
PL/SQL procedure successfully completed.

查看表与索引的消息

oracle@oracle10grac1 ~]$ dbtable orcl1 htz1

OWNER      TABLENAME  TABLESPACE ANALYZED               BLOCKS CACHE BUFFER_POO  INI_TRANS PAR TEMP BYTES        PCT_FREE   PCT_USED   NUM_ROWS LENGTH(A.DEGREE)
---------- ---------- ---------- -------------------- -------- ----- ---------- ---------- --- ---- ---------- ---------- ---------- ---------- ----------------
LUOPING    HTZ1       USERS                                        N DEFAULT             1 NO  N    12M                10                                     10

OWNER      TABLENAME  TABLESPACENAME  INDEXNAME       ANALYZED               C_FACTOR STATUS   BUFFER_POO INDEX_TYPE   NUM_ROWS COLUMNNAME      COLUMNPOST
---------- ---------- --------------- --------------- -------------------- ---------- -------- ---------- ---------- ---------- --------------- ----------
LUOPING    htz1       USERS           HTZ1_A                                          VALID    DEFAULT    NORMAL                A                        1
[oracle@oracle10grac1 ~]$ dbtable orcl1 htz1

OWNER      TABLENAME  TABLESPACE ANALYZED               BLOCKS CACHE BUFFER_POO  INI_TRANS PAR TEMP BYTES        PCT_FREE   PCT_USED   NUM_ROWS LENGTH(A.DEGREE)
---------- ---------- ---------- -------------------- -------- ----- ---------- ---------- --- ---- ---------- ---------- ---------- ---------- ----------------
LUOPING    HTZ1       USERS                                        N DEFAULT             1 NO  N    13M                10                                     10

OWNER      TABLENAME  TABLESPACENAME  INDEXNAME       ANALYZED               C_FACTOR STATUS   BUFFER_POO INDEX_TYPE   NUM_ROWS COLUMNNAME      COLUMNPOST
---------- ---------- --------------- --------------- -------------------- ---------- -------- ---------- ---------- ---------- --------------- ----------
LUOPING    htz1       USERS           HTZ1_A                                          VALID    DEFAULT    NORMAL                A                        1
[oracle@oracle10grac1 ~]$ dbtable orcl1 htz2

OWNER      TABLENAME  TABLESPACE ANALYZED               BLOCKS CACHE BUFFER_POO  INI_TRANS PAR TEMP BYTES        PCT_FREE   PCT_USED   NUM_ROWS LENGTH(A.DEGREE)
---------- ---------- ---------- -------------------- -------- ----- ---------- ---------- --- ---- ---------- ---------- ---------- ---------- ----------------
LUOPING    HTZ2       USERS                                        N DEFAULT             1 NO  N    13M                10                                     10

OWNER      TABLENAME  TABLESPACENAME  INDEXNAME       ANALYZED               C_FACTOR STATUS   BUFFER_POO INDEX_TYPE   NUM_ROWS COLUMNNAME      COLUMNPOST
---------- ---------- --------------- --------------- -------------------- ---------- -------- ---------- ---------- ---------- --------------- ----------
LUOPING    htz2       USERS           HTZ2_A                                          VALID    DEFAULT    NORMAL                A                        1

对表进行分析：

[oracle@oracle10grac1 ~]$ dbmsstats orcl1 luoping htz1
[oracle@oracle10grac1 ~]$ dbmsstats orcl1 luoping htz2

[oracle@oracle10grac1 ~]$ dbtable orcl1 htz2

OWNER      TABLENAME  TABLESPACE ANALYZED               BLOCKS CACHE BUFFER_POO  INI_TRANS PAR TEMP BYTES        PCT_FREE   PCT_USED   NUM_ROWS LENGTH(A.DEGREE)
---------- ---------- ---------- -------------------- -------- ----- ---------- ---------- --- ---- ---------- ---------- ---------- ---------- ----------------
LUOPING    HTZ2       USERS      2012-05-30 23:36:00      1630     N DEFAULT             1 NO  N    13M                10               1001889               10

OWNER      TABLENAME  TABLESPACENAME  INDEXNAME       ANALYZED               C_FACTOR STATUS   BUFFER_POO INDEX_TYPE   NUM_ROWS COLUMNNAME      COLUMNPOST
---------- ---------- --------------- --------------- -------------------- ---------- -------- ---------- ---------- ---------- --------------- ----------
LUOPING    htz2       USERS           HTZ2_A          2012-05-30 23:36:10        3437 VALID    DEFAULT    NORMAL        1000000 A                        1
[oracle@oracle10grac1 ~]$ dbtable orcl1 htz1

OWNER      TABLENAME  TABLESPACE ANALYZED               BLOCKS CACHE BUFFER_POO  INI_TRANS PAR TEMP BYTES        PCT_FREE   PCT_USED   NUM_ROWS LENGTH(A.DEGREE)
---------- ---------- ---------- -------------------- -------- ----- ---------- ---------- --- ---- ---------- ---------- ---------- ---------- ----------------
LUOPING    HTZ1       USERS      2012-05-30 23:35:04      1630     N DEFAULT             1 NO  N    13M                10                999943               10

OWNER      TABLENAME  TABLESPACENAME  INDEXNAME       ANALYZED               C_FACTOR STATUS   BUFFER_POO INDEX_TYPE   NUM_ROWS COLUMNNAME      COLUMNPOST
---------- ---------- --------------- --------------- -------------------- ---------- -------- ---------- ---------- ---------- --------------- ----------
LUOPING    htz1       USERS           HTZ1_A          2012-05-30 23:35:25        3437 VALID    DEFAULT    NORMAL        1000000 A                        1

下面对两张表进行删除5W行记录：

 

SQL> delete from htz1 where rownum <50000;

49999 rows deleted.

SQL> delete from htz2 where rownum <50000;

49999 rows deleted.

SQL> commit;

查看表站用BLOCK的情况：

 

SQL> exec show_space('HTZ1','LUOPING');
Unformatted Blocks .....................              62
FS1 Blocks (0-25) ......................               0
FS2 Blocks (25-50) .....................               0
FS3 Blocks (50-75) .....................               1
FS4 Blocks (75-100).....................             128
Full Blocks        .....................           1,439
Total Blocks............................           1,664
Total Bytes.............................      13,631,488
Total MBytes............................              13
Unused Blocks...........................               0
Unused Bytes............................               0
Last Used Ext FileId....................               4
Last Used Ext BlockId...................           7,561
Last Used Block.........................             128

PL/SQL procedure successfully completed.

SQL> exec show_space('HTZ2','LUOPING');
Unformatted Blocks .....................              62
FS1 Blocks (0-25) ......................               0
FS2 Blocks (25-50) .....................               0
FS3 Blocks (50-75) .....................               1
FS4 Blocks (75-100).....................             128
Full Blocks        .....................           1,439
Total Blocks............................           1,664
Total Bytes.............................      13,631,488
Total MBytes............................              13
Unused Blocks...........................               0
Unused Bytes............................               0
Last Used Ext FileId....................               4
Last Used Ext BlockId...................           7,433
Last Used Block.........................             128

PL/SQL procedure successfully completed.

下面对表进行MOVE与SHRINK

 

SQL> alter table luoping.htz2 enable row movement;

Table altered.

SQL> alter table luoping.htz2  shrink space;
SQL> alter table luoping.htz1 move;

Table altered.

下面查看表LOCK消息：

在SHRINK的时候，产生了行级row-X (SX)，在MOVE的时候产生了表级exclusive (X)，如果是在生产环境一定得注意这个。

[oracle@oracle10grac1 dbmonitor]$ dblock orcl1
ACC1 SESSION LOCK STATUS!

    XIDUSN    XIDSLOT     XIDSQN SESSION_ID ORACLE_USE OBJECT_NAME          LOCKED_MODE   OBJECT_TYPE
---------- ---------- ---------- ---------- ---------- -------------------- ------------- --------------------
         7          1       1079        151 SYS        HTZ2                 row-X (SX)    TABLE

ACC1 SESSION LOCK STATUS!

    XIDUSN    XIDSLOT     XIDSQN SESSION_ID ORACLE_USE OBJECT_NAME          LOCKED_MODE   OBJECT_TYPE
---------- ---------- ---------- ---------- ---------- -------------------- ------------- --------------------
         5         39       1321        151 SYS        HTZ2                 row-X (SX)    TABLE

ACC1 SESSION LOCK STATUS!

no rows selected

ACC1 SESSION LOCK STATUS!

no rows selected

ACC1 SESSION LOCK STATUS!

    XIDUSN    XIDSLOT     XIDSQN SESSION_ID ORACLE_USE OBJECT_NAME          LOCKED_MODE   OBJECT_TYPE
---------- ---------- ---------- ---------- ---------- -------------------- ------------- --------------------
         6         19       1303        151 SYS        HTZ1                 exclusive (X) TABLE

下面查看表与索引的消息：

MOVE的时候表上的索引会自动的不可能用，因为表中的数据是基于ROWID，而在做MOVE的时候ROWID发生了变化。而SHRINK表的时候，会自己更新索引的消息。

[oracle@oracle10grac1 dbmonitor]$ dbtable orcl1 htz1

OWNER      TABLENAME  TABLESPACE ANALYZED               BLOCKS CACHE BUFFER_POO  INI_TRANS PAR TEMP BYTES        PCT_FREE   PCT_USED   NUM_ROWS LENGTH(A.DEGREE)
---------- ---------- ---------- -------------------- -------- ----- ---------- ---------- --- ---- ---------- ---------- ---------- ---------- ----------------
LUOPING    HTZ1       USERS      2012-05-30 23:54:42      1478     N DEFAULT             1 NO  N    12M                10                953347               10

OWNER      TABLENAME  TABLESPACENAME  INDEXNAME       ANALYZED               C_FACTOR STATUS   BUFFER_POO INDEX_TYPE   NUM_ROWS COLUMNNAME      COLUMNPOST
---------- ---------- --------------- --------------- -------------------- ---------- -------- ---------- ---------- ---------- --------------- ----------
LUOPING    htz1       USERS           HTZ1_A          2012-05-30 23:48:22        3328 UNUSABLE DEFAULT    NORMAL         950001 A                        1
[oracle@oracle10grac1 dbmonitor]$ dbtable orcl1 htz2

OWNER      TABLENAME  TABLESPACE ANALYZED               BLOCKS CACHE BUFFER_POO  INI_TRANS PAR TEMP BYTES        PCT_FREE   PCT_USED   NUM_ROWS LENGTH(A.DEGREE)
---------- ---------- ---------- -------------------- -------- ----- ---------- ---------- --- ---- ---------- ---------- ---------- ---------- ----------------
LUOPING    HTZ2       USERS      2012-05-30 23:48:00      1630     N DEFAULT             1 NO  N    11.5625M           10                945834               10

OWNER      TABLENAME  TABLESPACENAME  INDEXNAME       ANALYZED               C_FACTOR STATUS   BUFFER_POO INDEX_TYPE   NUM_ROWS COLUMNNAME      COLUMNPOST
---------- ---------- --------------- --------------- -------------------- ---------- -------- ---------- ---------- ---------- --------------- ----------
LUOPING    htz2       USERS           HTZ2_A          2012-05-30 23:48:10        3328 VALID    DEFAULT    NORMAL         950001 A

手动对相关索引rebuild online一次

下面查看表占用块的消息：

 

SQL> exec show_space('HTZ1','LUOPING');
Unformatted Blocks .....................               0
FS1 Blocks (0-25) ......................               0
FS2 Blocks (25-50) .....................               0
FS3 Blocks (50-75) .....................               0
FS4 Blocks (75-100).....................               0
Full Blocks        .....................           1,446
Total Blocks............................           1,536
Total Bytes.............................      12,582,912
Total MBytes............................              12
Unused Blocks...........................              58
Unused Bytes............................         475,136
Last Used Ext FileId....................               4
Last Used Ext BlockId...................           9,097
Last Used Block.........................              70

PL/SQL procedure successfully completed.

SQL> exec show_space('HTZ2','LUOPING');
Unformatted Blocks .....................               0
FS1 Blocks (0-25) ......................               0
FS2 Blocks (25-50) .....................               1
FS3 Blocks (50-75) .....................               0
FS4 Blocks (75-100).....................               0
Full Blocks        .....................           1,441
Total Blocks............................           1,480
Total Bytes.............................      12,124,160
Total MBytes............................              11
Unused Blocks...........................               6
Unused Bytes............................          49,152
Last Used Ext FileId....................               4
Last Used Ext BlockId...................           6,921
Last Used Block.........................              66

 

相关的值已经明显下降。