DB2删除完数据之后，如何释放LOB字段占用的空间

DB2中，删除包含LOB字段的数据之后，如何释放LOB所占用表空间？


在db2中，某个表包括LOB字段，删除完数据之后，如果对这个表REORG的方式不对，则LOB字段所占用的空间并不能得到释放。

测试： 

1. 新建一个表T2，并插入数据，观察到所占空间：DATA_OBJECT_L_SIZE为512byte, LOB_OBJECT_L_SIZE为131460

db2 "create table t2(id int, address clob (32 k)) in tbs2 long in LONGTBS"

//insert some data here

db2 "SELECT substr(TABSCHEMA,1,10) as TABSCHEMA, substr(TABNAME,1,10) as TABNAME, DATA_OBJECT_L_SIZE, LOB_OBJECT_L_SIZE FROM SYSIBMADM.ADMINTABINFO WHERE TABNAME='T2' "

TABSCHEMA  TABNAME    DATA_OBJECT_L_SIZE   LOB_OBJECT_L_SIZE
---------- ---------- -------------------- --------------------
MIAOQINGSO T2                          512               131460

  1 record(s) selected.

2.  删除完数据之后，发现DATA_OBJECT_L_SIZE和LOB_OBJECT_L_SIZE都没有变化：

db2 "delete from t2"

db2 "SELECT substr(TABSCHEMA,1,10) as TABSCHEMA, substr(TABNAME,1,10) as TABNAME, DATA_OBJECT_L_SIZE, LOB_OBJECT_L_SIZE FROM SYSIBMADM.ADMINTABINFO WHERE TABNAME='T2' "

TABSCHEMA  TABNAME    DATA_OBJECT_L_SIZE   LOB_OBJECT_L_SIZE
---------- ---------- -------------------- --------------------
MIAOQINGSO T2                          512               131460

  1 record(s) selected.

3.  reorg t2之后，发现DATA_OBJECT_L_SIZE变小，但LOB_OBJECT_L_SIZE仍然没有变化，如果观察LONGTBS表空间的话，发现free pages并没有增加：

db2 "reorg table t2"

db2 "SELECT substr(TABSCHEMA,1,10) as TABSCHEMA, substr(TABNAME,1,10) as TABNAME, DATA_OBJECT_L_SIZE, LOB_OBJECT_L_SIZE FROM SYSIBMADM.ADMINTABINFO WHERE TABNAME='T2' "

TABSCHEMA  TABNAME    DATA_OBJECT_L_SIZE   LOB_OBJECT_L_SIZE
---------- ---------- -------------------- --------------------
MIAOQINGSO T2                          256               131460

  1 record(s) selected.

4. 这是work as design的，因为没有reorg LOB字段，REROG的时候，加上LONGLOBDATA，问题解决：

db2 "reorg table t2 LONGLOBDATA"

db2 "SELECT substr(TABSCHEMA,1,10) as TABSCHEMA, substr(TABNAME,1,10) as TABNAME, DATA_OBJECT_L_SIZE, LOB_OBJECT_L_SIZE FROM SYSIBMADM.ADMINTABINFO WHERE TABNAME='T2' "

TABSCHEMA  TABNAME    DATA_OBJECT_L_SIZE   LOB_OBJECT_L_SIZE
---------- ---------- -------------------- --------------------
MIAOQINGSO T2                          256                  260

  1 record(s) selected.

参考资料：

https://www.ibm.com/support/knowledgecenter/SSEPGG_10.5.0/com.ibm.db2.luw.admin.cmd.doc/doc/r0001966.html  

也有一些例外情况，reorg时不需要加上LONGLOBDATA的，参考如下
http://www-01.ibm.com/support/docview.wss?uid=swg21661779