Dropping Very Large Table In Oracle

这是一张550G的大表，表上还包括了CLOB和BLOB对象；我们来观察下Oracle drop这样一个大表时的具体表现:

SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE    10.2.0.4.0      Production
TNS for Linux: Version 10.2.0.4.0 - Production
NLSRTL Version 10.2.0.4.0 - Production

SQL> show parameter recyclebin

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
recyclebin                           string      OFF

/* 为了避免被flashback table骚扰，关闭了recyclebin回收站功能 */

SQL> conn maclean/maclean
Connected.

SQL> col segment_name for a20
SQL> select segment_name,bytes/1024/1024/1024 "size in Gbytes"  from user_segments where segment_name='TV';

SEGMENT_NAME         size in Gbytes
-------------------- --------------
TV                           547.25

SQL> select NUM_ROWS,BLOCKS from user_tables where table_name='TV';

  NUM_ROWS     BLOCKS
---------- ----------
 859150100   65649786

SQL> desc tv;
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 OWNER                                              VARCHAR2(30)
 OBJECT_NAME                                        VARCHAR2(128)
 SUBOBJECT_NAME                                     VARCHAR2(30)
 OBJECT_ID                                          NUMBER
 DATA_OBJECT_ID                                     NUMBER
 OBJECT_TYPE                                        VARCHAR2(19)
 CREATED                                            DATE
 LAST_DDL_TIME                                      DATE
 TIMESTAMP                                          VARCHAR2(19)
 STATUS                                             VARCHAR2(7)
 TEMPORARY                                          VARCHAR2(1)
 GENERATED                                          VARCHAR2(1)
 SECONDARY                                          VARCHAR2(1)
 SPARE1                                             CLOB
 SPARE2                                             CLOB
 SPARE3                                             CLOB
 SPARE4                                             BLOB

/* 该大表包含CLOB、BLOB2种大对象，共859150100行数据，占用65649786个块
    其所在是一个大文件表空间(bigfile tablespace)，本地区间管理方式，区间大小统一为128MB
*/

SQL> col tablespace_name for a2
SQL> select relative_fno,header_block,owner,tablespace_name from dba_segments where segment_name='TV';
RELATIVE_FNO HEADER_BLOCK OWNER                          TA
------------ ------------ ------------------------------ --
        1024           82 MACLEAN                        BF

/* 因为是用bigfile tablespace技术，故数据段所在相对数据文件号为1024 */


SQL> col segment_name for a30
SQL> col owner for a10
SQL> select owner,segment_name,segment_type,header_block from dba_segments where relative_fno=1024;

OWNER      SEGMENT_NAME                   SEGMENT_TYPE       HEADER_BLOCK
---------- ------------------------------ ------------------ ------------
MACLEAN    TV                             TABLE                        82
MACLEAN    SYS_IL0000057409C00014$$       LOBINDEX                  32850
MACLEAN    SYS_IL0000057409C00015$$       LOBINDEX                  65618
MACLEAN    SYS_IL0000057409C00016$$       LOBINDEX                  98386
MACLEAN    SYS_IL0000057409C00017$$       LOBINDEX                 131154
MACLEAN    SYS_LOB0000057409C00014$$      LOBSEGMENT                16466
MACLEAN    SYS_LOB0000057409C00015$$      LOBSEGMENT                49234
MACLEAN    SYS_LOB0000057409C00016$$      LOBSEGMENT                82002
MACLEAN    SYS_LOB0000057409C00017$$      LOBSEGMENT               114770

9 rows selected.

/* 该bigfile数据文件1024上的数据段如上包括TV表本身以及LOB对象和LOB索引 */

SQL> select current_scn from v$database;
CURRENT_SCN
-----------
   44989856

/* 获取当前scn以便闪回数据库 */

SQL> alter session set events '10046 trace name context forever,level 12';
Session altered.

SQL> set timing on;

SQL> drop table tv;
Table dropped.
Elapsed: 00:00:01.21

/* 虽然是550G的大表，但drop也仅耗时1.21s再次证明了drop仅仅是修改数据字典 */

通过tkprof分析的trace文件信息:
drop table tv

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.12       0.29         28          9      30163           0
Fetch        0      0.00       0.00          0          0          0           0
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        2      0.12       0.29         28          9      30163           0

Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: 64

Elapsed times include waiting on following events:
  Event waited on                             Times   Max. Wait  Total Waited
  ----------------------------------------   Waited  ----------  ------------
  db file sequential read                        28        0.00          0.00
  rdbms ipc reply                                10        0.01          0.06
  reliable message                                8        0.00          0.00
  enq: RO - fast object reuse                     8        0.00          0.00
  write complete waits                            9        0.04          0.10
  log file sync                                   1        0.00          0.00
  SQL*Net message to client                       1        0.00          0.00
  SQL*Net message from client                     1        7.22          7.22

以上可以看到少量的等待事件，drop要求所被drop对象先做object对象级别的checkpoint检查点，以便将脏块写出；故而会出现Write complete waits(A user wants to modify a block that is part of DBWR