在高并发、高负载的情况下，如何给表添加字段并设置DEFAULT值？
作者：互联网
在高并发、高负载的情况下，如何给表添加字段并设置DEFAULT值？
在Oracle 12c之前，当Oracle表数据量上亿时，对表执行“ALTER TABLE XXX ADD COLUMN_XX VARCHAR2(2) DEFAULT 'XXX';”操作时，效率及安全性是必须要考虑的因素。若直接执行，则会在该过程中给表加上6级表锁，也就是连查询都需要等待，这在生产库上是相当危险的操作。因为Oracle在执行上述操作过程中，不仅要更新数据字典，还会刷新全部的记录，并且会使得Undo表空间暴涨，所以，正确的做法是将更新数据字典和更新字段值分开。
例如，表LKILL.T_KILL约有4500W的数据，直接添加一个字段C_LHR需要花费21分钟，如下所示：
12:20:17 SYS@RACLHR2> ALTER TABLE LKILL.T_KILL ADD C_LHR VARCHAR2(100) DEFAULT 'LHR';
Table altered.
Elapsed: 00:21:58.53
若修改为如下的方式，则可以显著提高这个操作的性能，但表中原有的记录对于新添加的列为空，新增记录默认值会设置为LHR，那么原有记录的默认值就需要在系统空闲的时候进行批量更新、批量提交或采用系统包DBMS_PARALLEL_EXECUTE来更新，这样不至于大批量锁表，请参考本书中分批更新的部分【 REF _Ref24783 \n \h 3.1.10.5 REF _Ref24783 \h 分批插入、分批更新、分批删除、分批提交】。如下所示：
12:42:17 SYS@RACLHR2> ALTER TABLE LKILL.T_KILL ADD A_LHR VARCHAR2(100);
Table altered.
Elapsed: 00:00:00.35
13:53:54 SYS@RACLHR2> ALTER TABLE LKILL.T_KILL MODIFY A_LHR VARCHAR2(100) DEFAULT 'LHR';
Table altered.
Elapsed: 00:00:00.06
需要注意的是，从Oracle 11g开始，当添加一个带有默认值的非空列时（注意2个条件，NOT NULL和默认值），Oracle不会使用这个默认值来物理更新现有存在的行，Oracle只会存储这个新列元数据（NOT NULL约束和DEFAULT默认值），从而使得对该表的添加带有默认值的非空列操作可以在瞬间完成。当然，从表中检索该列时，会有部分的NVL函数代价。具体的细微差别可以通过10046事件来分析，这里不再详细解析。
从Oracle 12c开始，支持具有默认值的空列的添加列的DDL语句优化，即如下2条SQL语句的效率是一样的，也不存在锁表的现象了：
ALTER TABLE LKILL.T_KILL ADD A_LHR VARCHAR2(100);
ALTER TABLE LKILL.T_KILL ADD A_LHR VARCHAR2(100) NOT NULL;
示例如下所示：
LHR@OCPLHR1> select * from v$version where rownum<=1;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
LHR@OCPLHR1> set time on
16:59:00 LHR@OCPLHR1> set timing on
16:59:08 LHR@OCPLHR1> CREATE TABLE t1 AS
16:59:21 2 SELECT ROWNUM N1,
16:59:21 3 TRUNC((ROWNUM - 1) / 3) N2,
16:59:21 4 TRUNC(DBMS_RANDOM.VALUE(ROWNUM, ROWNUM * 10)) N3,
16:59:21 5 DBMS_RANDOM.STRING('U', 10) cl
16:59:21 6 FROM DUAL
16:59:21 7 CONNECT BY LEVEL <= 200000;
Table created.
Elapsed: 00:00:05.72
16:59:45 LHR@OCPLHR1> SELECT d.bytes FROM user_segments d WHERE d.segment_name='T1';
BYTES
----------
7340032
Elapsed: 00:00:00.09
17:01:00 LHR@OCPLHR1> ALTER TABLE t1 ADD c_ddl NUMBER DEFAULT 666 ;
Table altered.
Elapsed: 00:00:25.29
17:02:07 LHR@OCPLHR1> SELECT d.bytes FROM user_segments d WHERE d.segment_name='T1';
BYTES
----------
8388608
Elapsed: 00:00:00.01
17:02:13 LHR@OCPLHR1> ALTER TABLE t1 ADD c_ddl2 NUMBER DEFAULT 888 not null;
Table altered.
Elapsed: 00:00:00.08
17:02:37 LHR@OCPLHR1> SELECT d.bytes FROM user_segments d WHERE d.segment_name='T1';
BYTES
----------
8388608
Elapsed: 00:00:00.01
可以看出，在Oracle 11g中，加了NOT NULL约束的SQL语句，可以在瞬间完成添加列的操作，而只设置了默认值的SQL语句使用了25秒的时间。另外，加了NOT NUL约束的SQL语句执行完毕后，表的大小没有变化，这也说明了Oracle并没有做物理更新。
下面查看其执行计划，注意在这里不要使用“SET AUTOT ON”的方式，否则不能看到其真实的执行计划：
17:05:30 LHR@OCPLHR1> SELECT COUNT(*) FROM t1 WHERE c_ddl2=888;
COUNT(*)
----------
200000
Elapsed: 00:00:00.02
17:05:39 LHR@OCPLHR1> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
-------------------------------------------------------------
SQL_ID bq50v8z914juk, child number 0
-------------------------------------
SELECT COUNT(*) FROM t1 WHERE c_ddl2=888
Plan hash value: 3724264953
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 282 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
|* 2 | TABLE ACCESS FULL| T1 | 199K| 2530K| 282 (2)| 00:00:04 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(NVL("C_DDL2",888)=888)
Note
-----
- dynamic sampling used for this statement (level=2)
23 rows selected.
17:08:55 LHR@OCPLHR1> SELECT * FROM t1 WHERE rownum<=1;
N1 N2 N3 CL C_DDL C_DDL2
---------- ---------- ---------- ---------- ---------- ----------
1 0 8 XYGGZXRRYR 666 888
可以看到，在谓词部分出现了NVL函数。所以，Oracle认为C_DDL2列是空列。
下面测试是否可以使用索引：
17:29:24 LHR@OCPLHR1> CREATE INDEX idx_c_ddl2 ON t1(c_ddl2);
Index created.
Elapsed: 00:00:00.71
17:31:08 LHR@OCPLHR1> update t1 set c_ddl2='8881' where rownum<=1;
1 row updated.
Elapsed: 00:00:00.05
17:31:13 LHR@OCPLHR1> commit;
Commit complete.
Elapsed: 00:00:00.00
17:31:16 LHR@OCPLHR1> SELECT * FROM t1 WHERE c_ddl2=8881;
N1 N2 N3 CL C_DDL C_DDL2
---------- ---------- ---------- ---------- ---------- ----------
1 0 8 XYGGZXRRYR 666 8881
Elapsed: 00:00:00.01
17:31:24 LHR@OCPLHR1> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------
SQL_ID 0sm5s7zkvycrq, child number 0
-------------------------------------
SELECT * FROM t1 WHERE c_ddl2=8881
Plan hash value: 1464185165
------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 2 (100)| |
| 1 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 34 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_C_DDL2 | 1 | | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("C_DDL2"=8881)
19 rows selected.
Elapsed: 00:00:00.11
令人惊喜的是，使用了索引。
下面看看在Oracle 12c中的执行情况：
LHR@lhr121> set line 120
LHR@lhr121> select * from v$version where rownum<=1;
BANNER CON_ID
-------------------------------------------------------------------------------- ----------
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production 0
Elapsed: 00:00:00.00
LHR@lhr121> CREATE TABLE t1 AS
2 SELECT ROWNUM N1,
3 TRUNC((ROWNUM - 1) / 3) N2,
4 TRUNC(DBMS_RANDOM.VALUE(ROWNUM, ROWNUM * 10)) N3,
DBMS_RANDOM.STRING('U', 10) cl
6 FROM DUAL
7 CONNECT BY LEVEL <= 100000;
Table created.
Elapsed: 00:00:09.41
LHR@lhr121> SELECT d.bytes FROM user_segments d WHERE d.segment_name='T1';
BYTES
----------
4194304
Elapsed: 00:00:00.33
LHR@lhr121> ALTER TABLE t1 ADD c_ddl NUMBER DEFAULT 666 ;
Table altered.
Elapsed: 00:00:00.65
LHR@lhr121> SELECT d.bytes FROM user_segments d WHERE d.segment_name='T1';
BYTES
----------
4194304
Elapsed: 00:00:00.14
LHR@lhr121> ALTER TABLE t1 ADD c_ddl2 NUMBER DEFAULT 888 not null;
Table altered.
Elapsed: 00:00:00.15
LHR@lhr121> SELECT d.bytes FROM user_segments d WHERE d.segment_name='T1';
BYTES
----------
4194304
Elapsed: 00:00:00.09
LHR@lhr121> SELECT COUNT(*) FROM t1 WHERE c_ddl2=888;
COUNT(*)
----------
100000
Elapsed: 00:00:00.02
LHR@lhr121> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------
SQL_ID bq50v8z914juk, child number 1
-------------------------------------
SELECT COUNT(*) FROM t1 WHERE c_ddl2=888
Plan hash value: 3724264953
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 122 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
|* 2 | TABLE ACCESS FULL| T1 | 100K| 1269K| 122 (1)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(NVL("C_DDL2",888)=888)
Note
-----
- statistics feedback used for this statement
23 rows selected.
Elapsed: 00:00:00.05
LHR@lhr121> SELECT COUNT(*) FROM t1 WHERE c_ddl=666;
COUNT(*)
----------
100000
Elapsed: 00:00:00.04
LHR@lhr121> select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------
SQL_ID dph2gfp6f0jja, child number 1
-------------------------------------
SELECT COUNT(*) FROM t1 WHERE c_ddl=666
Plan hash value: 3724264953
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 122 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
|* 2 | TABLE ACCESS FULL| T1 | 1000 | 13000 | 122 (1)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(DECODE(TO_CHAR(SYS_OP_VECBIT("SYS_NC00005$",0)),NULL,NVL("
C_DDL",666),'0',NVL("C_DDL",666),'1',"C_DDL")=666)
20 rows selected.
Elapsed: 00:00:00.12
LHR@lhr121> SELECT d.column_name, d.column_id,d.hidden_column,d.virtual_column FROM Dba_Tab_Cols d WHERE d.table_name='T1' order by column_id;
COLUMN_NAME COLUMN_ID HID VIR
--------------- ---------- --- ---
N1 1 NO NO
N2 2 NO NO
N3 3 NO NO
CL 4 NO NO
C_DDL 5 NO NO
C_DDL2 6 NO NO
SYS_NC00005$ YES NO
7 rows selected.
Elapsed: 00:00:00.32
LHR@lhr121>
从示例可以清楚地看到，在Oracle 12c中，添加具有默认值的DDL优化已扩展到包括默认值的空列。Oracle使用了一个未公开的函数SYS_OP_VECBIT和新的隐藏列SYS_NC00005$，因为该列没有被物理更新。
& 说明：
有关批量更新和DBMS_PARALLEL_EXECUTE的使用更详细的内容可以参考我的BLOG：① http://blog.itpub.net/26736162/viewspace-2140626/ ②http://blog.itpub.net/26736162/viewspace-1684396
标签：给表,00,DEFAULT,Elapsed,字段,LHR,----------,TABLE,SELECT 来源： https://blog.51cto.com/lhrbest/2702888