deallocate unused :僅適用于釋放HWM高水位以上的空間，而無(wú)法釋放高水位以下的空間；比如對(duì)表預(yù)分配的空間

創(chuàng)新互聯(lián)公司主要從事網(wǎng)頁(yè)設(shè)計(jì)、PC網(wǎng)站建設(shè)（電腦版網(wǎng)站建設(shè)）、wap網(wǎng)站建設(shè)（手機(jī)版網(wǎng)站建設(shè)）、成都響應(yīng)式網(wǎng)站建設(shè)公司、程序開(kāi)發(fā)、網(wǎng)站優(yōu)化、微網(wǎng)站、小程序定制開(kāi)發(fā)等，憑借多年來(lái)在互聯(lián)網(wǎng)的打拼，我們?cè)诨ヂ?lián)網(wǎng)網(wǎng)站建設(shè)行業(yè)積累了豐富的做網(wǎng)站、成都做網(wǎng)站、網(wǎng)站設(shè)計(jì)、網(wǎng)絡(luò)營(yíng)銷(xiāo)經(jīng)驗(yàn)，集策劃、開(kāi)發(fā)、設(shè)計(jì)、營(yíng)銷(xiāo)、管理等多方位專(zhuān)業(yè)化運(yùn)作于一體。

使用說(shuō)明和方法，官方文檔有說(shuō)明，如下：

Use the deallocate_unused_clause to explicitly deallocate unused space at the end of

a database object segment and make the space available for other segments in the

tablespace.

You can deallocate unused space using the following statements:

■ ALTER CLUSTER (see ALTER CLUSTER on page 10-5)

■ ALTER INDEX: to deallocate unused space from the index, an index partition, or an

index subpartition (see ALTER INDEX on page 10-78)

■ ALTER MATERIALIZED VIEW: to deallocate unused space from the overflow segment

of an index-organized materialized view (see ALTER MATERIALIZED VIEW on

page 11-3)

■ ALTER TABLE: to deallocate unused space from the table, a table partition, a table

subpartition, the mapping table of an index-organized table, the overflow segment

of an index-organized table, or a LOB storage segment (see ALTER TABLE on

page 12-2)

一、測(cè)試環(huán)境：

 Oracle11.2.0.4

 使用admin用戶(hù)在test表空間新建一張測(cè)試分區(qū)表，并提前對(duì)子分區(qū)預(yù)分配一些空間，然后插入少量的數(shù)據(jù)，最后使用：alter  table .....deallocate unused;來(lái)釋放未使用的空間

二、開(kāi)始測(cè)試

①：查看測(cè)試環(huán)境表空間使用情況：

SQL> set linesize 2500
SQL> set pagesize 300
SQL> select a.tablespace_name "表空間名稱(chēng)",100 - round((nvl(b.bytes_free, 0) / a.bytes_alloc) * 100, 2) "占用率(%)",round(a.bytes_alloc / 1024 / 1024, 2) "容量(M)",round(nvl(b.bytes_free, 0) / 1024 / 1024, 2) "空閑(M)",round((a.bytes_alloc - nvl(b.bytes_free, 0)) / 1024 / 1024, 2) "使用(M)",to_char(sysdate, 'yyyy-mm-dd hh34:mi:ss') "采樣時(shí)間" from (select f.tablespace_name,sum(f.bytes) bytes_alloc,sum(decode(f.autoextensible, 'YES', f.maxbytes, 'NO', f.bytes)) maxbytes from dba_data_files f group by tablespace_name) a,(select f.tablespace_name, sum(f.bytes) bytes_free from dba_free_space f group by tablespace_name) b where a.tablespace_name = b.tablespace_name order by 2 desc;
???????????????                ?????????(%)  ??????(M)  ??????(M)  ??????(M) ????????????
------------------------------ ------------ ---------- ---------- ---------- -------------------
SYSAUX                                94.41        510       28.5      481.5 2017-08-07 17:01:20
SYSTEM                                93.24        800      54.06     745.94 2017-08-07 17:01:20
USERS                                 26.25          5       3.69       1.31 2017-08-07 17:01:20
UNDOTBS1                              17.97        470     385.56      84.44 2017-08-07 17:01:20
TEST                                    .05      20480      20470         10 2017-08-07 17:01:20

SQL> show user
USER is "ADMIN"     ----測(cè)試用的用戶(hù)

②：創(chuàng)建測(cè)試分區(qū)表：test_emp  

SQL> create table test_emp (TMP_UPSTATE_CASEKEY char(14),TMP_NUM_STATUS_ID number(38),updated_date date)
  2  partition by range(TMP_NUM_STATUS_ID)
  3          (partition pt_1 values less than(1000000) ) nologging; 
Table created.

③：給pt_1分區(qū)表預(yù)分配10G的空間：

SQL> alter table test_EMP  modify PARTITION pt_1 ALLOCATE EXTENT (size 10240m);
Table altered.

④：收集該分區(qū)表的統(tǒng)計(jì)信息，并查看該表的block塊占用情況

SQL>  exec dbms_stats.gather_table_stats(ownname =>'ADMIN',tabname =>'TEST_EMP',cascade=>TRUE);
PL/SQL procedure successfully completed.
SQL>  select B.SEGMENT_NAME, B.blocks,B.blocks * 8096 / 1024 / 1024, A.BLOCKS,A.blocks * 8096 / 1024 / 1024, A.EMPTY_BLOCKS from user_tables a, USER_SEGMENTS B WHERE TABLE_NAME = 'TEST_EMP' AND A.TABLE_NAME = B.SEGMENT_NAME; 
SEGMENT_NAME                                                                          BLOCKS B.BLOCKS*8096/1024/1024     BLOCKS A.BLOCKS*8096/1024/1024 EMPTY_BLOCKS
--------------------------------------------------------------------------------- ---------- ----------------------- ---------- ----------------------- ------------
TEST_EMP                                                                             1315840              10159.5313          0                       0            0
SQL> SELECT segment_name, SUM(bytes) / 1024 / 1024 Mbytes  FROM dba_segments WHERE    PARTITION_NAME = 'PT_1' GROUP BY segment_name;
SEGMENT_NAME                                                                          MBYTES
--------------------------------------------------------------------------------- ----------
TEST_EMP                                                                               10288
BIN$VibHVCPfDL/gU8gCqMDDfw==$0                                                             8

---從上面可以看出，test_emp分區(qū)表的大小是10G，而占用的blocks有1315840個(gè)；

⑤：向分區(qū)表插入一些數(shù)據(jù)，并查看該表的大小

SQL> create or replace procedure proc_casekey_upstate
  2  as
  3    casekey char(14);
  4  begin
  5    for i in 1..10000 loop
  6      casekey := 'TMP'||lpad(i,7,0);   
  7      insert into test_emp values(casekey, 1, sysdate);
  8    end loop;
  9    commit;
 10  end;
 11  /
Procedure created.
SQL> exec proc_casekey_upstate;    ---執(zhí)行存儲(chǔ)過(guò)程插入數(shù)據(jù)
PL/SQL procedure successfully completed.
SQL> exec dbms_stats.gather_table_stats(ownname =>'ADMIN',tabname =>'TEST_EMP',cascade=>TRUE);   ---收集該表的統(tǒng)計(jì)信息
PL/SQL procedure successfully completed.
SQL> select B.SEGMENT_NAME, B.blocks,B.blocks * 8096 / 1024 / 1024, A.BLOCKS,A.blocks * 8096 / 1024 / 1024, A.EMPTY_BLOCKS from user_tables a, USER_SEGMENTS B WHERE TABLE_NAME = 'TEST_EMP' AND A.TABLE_NAME = B.SEGMENT_NAME; 
SEGMENT_NAME                                                                          BLOCKS B.BLOCKS*8096/1024/1024     BLOCKS A.BLOCKS*8096/1024/1024 EMPTY_BLOCKS
--------------------------------------------------------------------------------- ---------- ----------------------- ---------- ----------------------- ------------
TEST_EMP                                                                             1315840              10159.5313         46              .355163574            0
SQL> SELECT segment_name, SUM(bytes) / 1024 / 1024 Mbytes  FROM dba_segments WHERE    PARTITION_NAME = 'PT_1' GROUP BY segment_name;
SEGMENT_NAME                                                                          MBYTES
--------------------------------------------------------------------------------- ----------
TEST_EMP                                                                               10288
BIN$VibHVCPfDL/gU8gCqMDDfw==$0                                                             8

--注意：從上面可以看出，該表的大小是10G，但是該表占用blocks只有46個(gè)，顯然 有很多空間沒(méi)有被使用，

⑥：釋放未被使用的空間（我對(duì)該操作過(guò)了10046事件，在實(shí)際操作中只需執(zhí)行：alter table test_emp modify partition pt_1 deallocate unused; ）

SQL> alter session set tracefile_identifier='10046';
SQL> alter session set events '10046 trace name context forever,level 12';   ---開(kāi)啟10046事件
Session altered.
SQL> 
SQL>  alter table test_emp modify partition pt_1 deallocate unused;   ---執(zhí)行該命令釋放子分區(qū)未被使用的空間；
 
Table altered.
SQL> alter session set events '10046 trace name context off';   ---關(guān)閉10046事件
Session altered.
SQL> select value from v$diag_info where name='Default Trace File';
VALUE
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/opt/oracle/diag/rdbms/dbs/dbs/trace/dbs_ora_3263_10046.trc

⑦：驗(yàn)證空間是否被釋放：

SQL> select B.SEGMENT_NAME, B.blocks,B.blocks * 8096 / 1024 / 1024, A.BLOCKS,A.blocks * 8096 / 1024 / 1024, A.EMPTY_BLOCKS from user_tables a, USER_SEGMENTS B WHERE TABLE_NAME = 'TEST_EMP' AND A.TABLE_NAME = B.SEGMENT_NAME; 
SEGMENT_NAME                                                                          BLOCKS B.BLOCKS*8096/1024/1024     BLOCKS A.BLOCKS*8096/1024/1024 EMPTY_BLOCKS
--------------------------------------------------------------------------------- ---------- ----------------------- ---------- ----------------------- ------------
TEST_EMP                                                                                1024                 7.90625         46              .355163574            0

SQL> SELECT segment_name, SUM(bytes) / 1024 / 1024 Mbytes  FROM dba_segments WHERE    PARTITION_NAME = 'PT_1' GROUP BY segment_name;
SEGMENT_NAME                                                                          MBYTES
--------------------------------------------------------------------------------- ----------
TEST_EMP                                                                                  16
BIN$VibHVCPfDL/gU8gCqMDDfw==$0                                                             8
------------------------------ ------------ ---------- ---------- ---------- -------------------
SYSAUX                                94.44        510      28.38     481.63 2017-08-07 17:19:30
SYSTEM                                93.24        800      54.06     745.94 2017-08-07 17:19:30
USERS                                 26.25          5       3.69       1.31 2017-08-07 17:19:30
UNDOTBS1                               5.73        470     443.06      26.94 2017-08-07 17:19:30
TEST                                    .05      20480      20470         10 2017-08-07 17:19:30

從上面可以看出，該表的大小變成了16M，而blocks也有1024個(gè)；表未被使用的空間已經(jīng)釋放，而且表空間可用大小也已經(jīng)變大：