在Oracle中,可以在创建主键约束的时候自动创建唯一索引,也可以先创建唯一索引,然后再基于这个唯一索引来创建主键约束。后一种
在Oracle中,可以在创建主键约束的时候自动创建唯一索引,也可以先创建唯一索引,然后再基于这个唯一索引来创建主键约束。后一种方式有一个好处,在 需要对数据量比较大而且读写频繁的OLTP表创建主键约束的时候,可以先ONLINE的创建一个唯一的索引,然后再创建主键约束,这样可以减少对表的读写 阻塞。但这样就带来一个问题,第一种方式创建的索引在删除约束的时候索引会被自动删除,而第二种方式创建的索引在删除约束的时候不会自动删除,,需要删完约 束删索引,如果忘记了这个唯一索引的话,可能会带来跟想象不一样的结果。同时,Oracle针对这种情况提供了特殊的删除约束的方法,可以同时删除约束和 索引,就是:alter table bear drop constraint pk_bear drop index。在删除约束的最后加上删除索引的关键字。
但写这一大堆都不是这里要重点描述的内容,这里要写的是怎么知道已经创建好的索引到底是第一种方式创建的?还是第二种方式创建的呢?
在Oracle的SYS.IND$视图中有一个叫PROPERTY的字段,里面记录的就是每个索引对应的属性,这个字段的含义是在创建这个表的SQL中有定义的。在10G的版本中,可以到$Oracle_HOME/rdbms/admin/sql.bsp中查找ind$表的创建脚本;在11G的版本中,可以到相同的位置的dcore.bsp中查找。这些脚本都是创建系统核心表的脚本,很多字段在官方文档中没有注释的,可以来这里找找看。在11G中对PROPERTY字段的注释如下:
property number not null, /* immutable flags for life of the index */
/* unique : 0x01 */
/* partitioned : 0x02 */
/* reverse : 0x04 */
/* compressed : 0x08 */
/* functional : 0x10 */
/* temporary table index: 0x20 */
/* session-specific temporary table index: 0x40 */
/* index on embedded adt: 0x80 */
/* user said to check max length at runtime: 0x0100 */
/* domain index on IOT: 0x0200 */
/* join index : 0x0400 */
/* system managed domain index : 0x0800 */
/* The index was created by a constraint : 0x1000 */
/* The index was created by create MV : 0x2000 */
/* composite domain index : 0x8000 */
这个是典型的Oracle的表示方法,其中每个值表示一个含义,但是多个值是可以累加起来表示多个含义的。比如一个UNIQUE的REVERSE的索引,对应的值就会是0X05,PROPERTY中对应的是10进制的存放,就应该也是5。
可以用下面的SQL来查询主键跟索引的关系是第一种还是第二种:
SELECT DECODE(BITAND(PROPERTY, 4096), 4096, 'implicit', 'user-generated') GENERATION,
B.INDEX_NAME
FROM SYS.IND$ A, USER_INDEXES B, USER_OBJECTS C
WHERE B.UNIQUENESS = 'UNIQUE'
AND A.OBJ# = C.OBJECT_ID
AND B.INDEX_NAME = C.OBJECT_NAME
AND B.INDEX_NAME = 'INDEX_NAME';
上面的SQL就是说如果PROPERTY为4096,那么对应到16进制应该是0X1000,也就表示The index was created by a constraint,也就是第一种;其他都是第二种,这种情况下典型的值就是4097,也就是0X1000和0X01值的和。

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