在11g之前增加一个not null的字段非常慢，在11g之后就非常快了，我们先做一个测试，然后探究下原理。
SQL> select * from v$version;
BANNER----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bi
PL/SQL Release 10.2.0.1.0 - Production
CORE 10.2.0.1.0 Production
TNS for 64-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production
SQL> drop table test purge;
SQL> create table test as select * from dba_objects;
SQL> select count(*) from test;
COUNT(*)
----------
151203
SQL> set timing on
SQL> alter table test add col1 char(1000) DEFAULT 'LARGE COLUMN' not null;
已用时间: 00: 00: 09.48
SQL> SELECT SUM(BYTES)/1024/1024 ||'M' FROM user_segments WHERE segment_name = 'TEST';
SUM(BYTES)/1024/1024||'M'
-----------------------------------------
200M
在11g下：
SQL> select * from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production
PL/SQL Release 11.2.0.1.0 - Production
CORE 11.2.0.1.0 Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production
SQL> drop table test purge;
SQL> create table test as select * from dba_objects;
SQL> insert into test select * from dba_objects;
SQL> commit;
SQL> select count(*) from test;
COUNT(*)
----------
149012
SQL> set timing on
SQL> alter table test add col1 char(1000) DEFAULT 'LARGE COLUMN' not null;
已用时间: 00: 00: 00.07
SUM(BYTES)/1024/1024||'M'
-----------------------------------------
18M
探究原理：dump block可以发现，在10g下产生了很多行迁移和行链接。而在11g中就是一个值，在官方文档中说是Oracle通过在数据字典中记录DEFAULT值，避免了繁重的更新操作。
在dump block看看：
alter session set tracefile_identifier = 'gg_test';
select rowid,
dbms_rowid.rowid_object(rowid) object_id,
dbms_rowid.rowid_relative_fno(rowid) file_id,
dbms_rowid.rowid_block_number(rowid) block_id,
dbms_rowid.rowid_row_number(rowid) num
from test where rownum <5;
alter system dump datafile 5 block 1465683;
10g下：
tl: 9 fb: --H----- lb: 0x2 cc: 0
nrid: 0x0181e36b.1
tab 0, row 2, @0x1b3a
tl: 1076 fb: --H-FL-- lb: 0x2 cc: 14
col 0: [ 3] 53 59 53
col 1: [ 4] 43 4f 4e 24
col 2: *NULL*
col 3: [ 2] c1 1d
col 4: [ 2] c1 1d
col 5: [ 5] 54 41 42 4c 45
col 6: [ 7] 78 69 08 1e 0e 33 19
col 7: [ 7] 78 69 08 1e 0f 31 37
col 8: [19] 32 30 30 35 2d 30 38 2d 33 30 3a 31 33 3a 35 30 3a 32 34
col 9: [ 5] 56 41 4c 49 44
col 10: [ 1] 4e
col 11: [ 1] 4e
col 12: [ 1] 4e
col 13: [1000]
4c 41 52 47 45 20 43 4f 4c 55 4d 4e 20 20 20 20 20 20 20 20 20 20 20 20 20
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
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20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
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20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
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20