deallocate unused :仅适用于释放HWM高水位以上的空间，而无法释放高水位以下的空间；比如对表预分配的空间
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使用说明和方法，官方文档有说明，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、测试环境：
Oracle11.2.0.4
使用admin用户在test表空间新建一张测试分区表，并提前对子分区预分配一些空间，然后插入少量的数据，最后使用：alter table .....deallocate unused;来释放未使用的空间
二、开始测试
①：查看测试环境表空间使用情况：
SQL> set linesize 2500 SQL> set pagesize 300 SQL> select a.tablespace_name "表空间名称",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) "空闲(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') "采样时间" 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" ----测试用的用户
②：创建测试分区表：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：给pt_1分区表预分配10G的空间：
SQL> alter table test_EMP modify PARTITION pt_1 ALLOCATE EXTENT (size 10240m); Table altered.
④：收集该分区表的统计信息，并查看该表的block块占用情况
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==$0 8
---从上面可以看出，test_emp分区表的大小是10G，而占用的blocks有1315840个；
⑤：向分区表插入一些数据，并查看该表的大小
SQL> create or replace procedure proc_casekey_upstate 2 as 3 casekey char(14); 4 begin 5 for i in 1..10000 loop 6 casekey := 'TMP'||lpad(i,7,0); 7 insert into test_emp values(casekey, 1, sysdate); 8 end loop; 9 commit; 10 end; 11 / Procedure created. SQL> exec proc_casekey_upstate; ---执行存储过程插入数据 PL/SQL procedure successfully completed. SQL> exec dbms_stats.gather_table_stats(ownname =>'ADMIN',tabname =>'TEST_EMP',cascade=>TRUE); ---收集该表的统计信息 PL/SQL procedure successfully completed. SQL> select B.SEGMENT_NAME, B.blocks,B.blocks * 8096 / 1024 / 1024, A.BLOCKS,A.blocks * 8096 / 1024 / 1024, A.EMPTY_BLOCKS from user_tables a, USER_SEGMENTS B WHERE TABLE_NAME = 'TEST_EMP' AND A.TABLE_NAME = B.SEGMENT_NAME; SEGMENT_NAME BLOCKS B.BLOCKS*8096/1024/1024 BLOCKS A.BLOCKS*8096/1024/1024 EMPTY_BLOCKS --------------------------------------------------------------------------------- ---------- ----------------------- ---------- ----------------------- ------------ TEST_EMP 1315840 10159.5313 46 .355163574 0 SQL> SELECT segment_name, SUM(bytes) / 1024 / 1024 Mbytes FROM dba_segments WHERE PARTITION_NAME = 'PT_1' GROUP BY segment_name; SEGMENT_NAME MBYTES --------------------------------------------------------------------------------- ---------- TEST_EMP 10288 BIN$VibHVCPfDL/gU8gCqMDDfw==$0 8
--注意：从上面可以看出，该表的大小是10G，但是该表占用blocks只有46个，显然 有很多空间没有被使用，
⑥：释放未被使用的空间（我对该操作过了10046事件，在实际操作中只需执行：alter table test_emp modify partition pt_1 deallocate unused; ）
SQL> alter session set tracefile_identifier='10046'; SQL> alter session set events '10046 trace name context forever,level 12'; ---开启10046事件 Session altered. SQL> SQL> alter table test_emp modify partition pt_1 deallocate unused; ---执行该命令释放子分区未被使用的空间； Table altered. SQL> alter session set events '10046 trace name context off'; ---关闭10046事件 Session altered. SQL> select value from v$diag_info where name='Default Trace File'; VALUE -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- /opt/oracle/diag/rdbms/dbs/dbs/trace/dbs_ora_3263_10046.trc
⑦：验证空间是否被释放：
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
从上面可以看出，该表的大小变成了16M，而blocks也有1024个；表未被使用的空间已经释放，而且表空间可用大小也已经变大：
