Oracle deallocate unused釋放高水位空間
deallocate unused :僅適用於釋放HWM高水位以上的空間，而無法釋放高水位以下的空間；比如對錶預分配的空間
使用說明和方法，官方文件有說明，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、測試環境：
使用admin使用者在test表空間新建一張測試分割槽表，並提前對子分割槽預分配一些空間，然後插入少量的資料，最後使用：alter table .....deallocate unused;來釋放未使用的空間
二、開始測試
①：檢視測試環境表空間使用情況：
SQL> set linesize 2500
SQL> set pagesize 300
SQL> select a.tablespace_name "表空間名稱",100 - round((nvl(b.bytes_free, 0) / a.bytes_alloc) * 100, 2) "佔用率(%)",round(a.bytes_alloc / 1024 / 1024, 2) "容量(M)",round(nvl(b.bytes_free, 0) / 1024 / 1024, 2) "空閒(M)",round((a.bytes_alloc - nvl(b.bytes_free, 0)) / 1024 / 1024, 2) "使用(M)",to_char(sysdate, 'yyyy-mm-dd hh24:mi:ss') "取樣時間" 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" ----測試用的使用者
②：建立測試分割槽表：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分割槽表預分配10G的空間：
SQL> alter table test_EMP modify PARTITION pt_1 ALLOCATE EXTENT (size 10240m);
Table altered.
④：收集該分割槽表的統計資訊，並檢視該表的block塊佔用情況
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==$0 8
---從上面可以看出，test_emp分割槽表的大小是10G，而佔用的blocks有1315840個；
⑤：向分割槽表插入一些資料，並檢視該表的大小
SQL> create or replace procedure proc_casekey_upstate
2 as
3 casekey char(14);
4 begin
5 for i in 1..10000 loop
6 casekey := 'TMP'||lpad(i,7,0);
7 insert into test_emp values(casekey, 1, sysdate);
8 end loop;
9 commit;
10 end;
11 /
Procedure created.
SQL> exec proc_casekey_upstate; ---執行儲存過程插入資料
PL/SQL procedure successfully completed.
SQL> exec dbms_stats.gather_table_stats(ownname =>'ADMIN',tabname =>'TEST_EMP',cascade=>TRUE); ---收集該表的統計資訊
PL/SQL procedure successfully completed.
SQL> select B.SEGMENT_NAME, B.blocks,B.blocks * 8096 / 1024 / 1024, A.BLOCKS,A.blocks * 8096 / 1024 / 1024, A.EMPTY_BLOCKS from user_tables a, USER_SEGMENTS B WHERE TABLE_NAME = 'TEST_EMP' AND A.TABLE_NAME = B.SEGMENT_NAME;
SEGMENT_NAME BLOCKS B.BLOCKS*8096/1024/1024 BLOCKS A.BLOCKS*8096/1024/1024 EMPTY_BLOCKS
--------------------------------------------------------------------------------- ---------- ----------------------- ---------- ----------------------- ------------
TEST_EMP 1315840 10159.5313 46 .355163574 0
SQL> SELECT segment_name, SUM(bytes) / 1024 / 1024 Mbytes FROM dba_segments WHERE PARTITION_NAME = 'PT_1' GROUP BY segment_name;
SEGMENT_NAME MBYTES
--------------------------------------------------------------------------------- ----------
TEST_EMP 10288
BIN$VibHVCPfDL/gU8gCqMDDfw==$0 8
--注意：從上面可以看出，該表的大小是10G，但是該表佔用blocks只有46個，顯然 有很多空間沒有被使用，
⑥：釋放未被使用的空間（我對該操作過了10046事件，在實際操作中只需執行：alter table test_emp modify partition pt_1 deallocate unused; ）
SQL> alter session set tracefile_identifier='10046';
SQL> alter session set events '10046 trace name context forever,level 12'; ---開啟10046事件
Session altered.
SQL>
SQL> alter table test_emp modify partition pt_1 deallocate unused; ---執行該命令釋放子分割槽未被使用的空間；
Table altered.
SQL> alter session set events '10046 trace name context off'; ---關閉10046事件
Session altered.
SQL> select value from v$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；表未被使用的空間已經釋放，而且表空間可用大小也已經變大：