| 1234567891011121314151617181920 | SQL>select *from v$version;BANNER--------------------------------------------------------------------------------OracleDatabase 11g Enterprise Edition Release 11.2.0.1.0 - Production PL/SQL Release 11.2.0.1.0 - ProductionCORE 11.2.0.1.0 ProductionTNSfor Linux: Version 11.2.0.1.0 - Production NLSRTL Version 11.2.0.1.0 - Production SQL>create table test ( 2 id int, 3 name varchar2(20), 4 constraint pk_testprimary key(id)) 5 tablespace users; Table created. SQL>select constraint_name, constraint_typefrom user_constraints;CONSTRAINT_NAME C------------------------------ -PK_TEST P |
在test表中，我们指定了ID列作为主键，Oracle数据库会自动创建一个同名的唯一索引：
| 1234567 | SQL>select index_name, index_type, uniqueness, tablespace_name 2 from user_indexes 3 where table_owner='SCOTT' 4 and table_name ='TEST';INDEX_NAME INDEX_TYPE UNIQUENES TABLESPACE_NAME-------------------- -------------------- --------- ------------------------------PK_TEST NORMAL UNIQUE USERS |
此时，如果我们再试图在ID列上创建一个唯一索引，Oracle会报错，因为该列上已经存在一个唯一索引：
| 12345 | SQL>create unique index idx_test_ukon test(id);create unique index idx_test_ukon test(id) *ERRORat line 1:ORA-01408: suchcolumn list already indexed |
即使创建非唯一索引也不行：
| 12345 | SQL>create index idx_test_idon test(id);create index idx_test_idon test(id) *ERRORat line 1:ORA-01408: suchcolumn list already indexed |
那么唯一键约束的情况是怎样的呢？
| 12345678910111213 | SQL>drop table test purge; Table dropped. SQL>create table test( 2 id int, 3 name varchar2(20), 4 constraint uk_testunique(id));Table created. SQL>select constraint_name, constraint_typefrom user_constraints;CONSTRAINT_NAME C------------------------------ -UK_TEST U |
查看此时的索引情况：
| 1234567 | SQL>select index_name, index_type, uniqueness, tablespace_name 2 from user_indexes 3 where table_owner='SCOTT' 4 and table_name ='TEST';INDEX_NAME INDEX_TYPE UNIQUENES TABLESPACE_NAME-------------------- -------------------- --------- ------------------------------UK_TEST NORMAL UNIQUE USERS |
Oracle同样自动创建了一个同名的唯一索引，而且也不允许再在此列上创建唯一索引或非唯一索引。 我们知道，主键约束要求列值非空（NOT NULL），那么唯一键约束是否也要求非空呢？
| 123456789101112131415 | SQL>insert into testvalues(1,'Sally');1 row created. SQL>insert into testvalues(null,'Tony');1 row created. SQL>insert into testvalues(null,'Jack');1 row created. SQL>select *from test; IDNAME---------- -------------------- 1 Sally Tony Jack |
从实验结果来看，唯一键约束并没有非空要求。 接下来我们看看唯一索引对列值的非空要求有什么不同。
| 1234567891011121314151617181920212223242526 | SQL>drop table test purge; Table dropped. SQL>create table test( 2 id int, 3 name varchar2(20));Table created. SQL>create unique index idx_test_idon test (id);Index created. SQL>insert into testvalues(1,'Sally');1 row created. SQL>insert into testvalues(null,'Tony');1 row created. SQL>insert into testvalues(null,'Jack');1 row created. SQL>select *from test; IDNAME---------- -------------------- 1 Sally Tony Jack |
通过实验，我们看出唯一索引与唯一键约束一样对列值非空不做要求。 如果我们让主键约束或者唯一键约束失效，Oracle自动创建的唯一索引是否会受到影响？
| 1234567891011121314151617181920212223242526 | SQL>drop table test purge; Table dropped.SQL>create table test( 2 id int, 3 name varchar2(20), 4 constraint uk_testunique(id));Table created. SQL>select index_name, index_type, uniqueness from user_indexes;INDEX_NAME INDEX_TYPE UNIQUENES------------------------------ --------------------------- ---------UK_TEST NORMAL UNIQUE SQL>alter table test disable constraint uk_test;Table altered. SQL>select index_name, index_type, uniqueness from user_indexes;no rows selected SQL>alter table test disable constraint uk_test;Table altered. SQL>select index_name, index_type, uniqueness from user_indexes;INDEX_NAME INDEX_TYPE UNIQUENES------------------------------ --------------------------- ---------UK_TEST NORMAL UNIQUE |
当主键约束或者唯一键约束失效时，Oracle会标记隐式创建的唯一索引为删除状态。=====================================================Using Nonunique Indexes to Enforce UniquenessYou can use an existing nonunique index on a table to enforce uniqueness, either for UNIQUE constraints or the unique aspect of a PRIMARY KEY constraint. The advantage of this approach is that the index remains available and valid when the constraint is disabled. Therefore, enabling a disabled UNIQUE or PRIMARY KEY constraint does not require rebuilding the unique index associated with the constraint. This can yield significant time savings on enable operations for large tables.Using a nonunique index to enforce uniqueness also lets you eliminate redundant indexes. You do not need a unique index on a primary key column if that column is included as the prefix of a composite index. You can use the existing index to enable and enforce the constraint. You also save significant space by not duplicating the index. However, if the existing index is partitioned, then the partitioning key of the index must also be a subset of the UNIQUE key; otherwise, Oracle Database creates an additional unique index to enforce the constraint.=====================================================如果我们先创建唯一索引，再创建主键或者唯一键约束，情况又会怎样呢？
| 1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677787980818283848586878889909192939495969798 | SQL>drop table test purge; Table dropped. SQL>create table test( 2 id int, 3 name varchar2(20));Table created. SQL>create unique index idx_test_idon test (id);Index created. SQL>select index_name, index_type, uniqueness 2 from user_indexes 3 where table_owner ='SCOTT' 4 and table_name ='TEST';INDEX_NAME INDEX_TYPE UNIQUENES------------------------------ --------------------------- ---------IDX_TEST_ID NORMAL UNIQUE SQL>alter table testadd constraint uk_testunique (id);Table altered. SQL>select index_name, index_type, uniqueness 2 from user_indexes 3 where table_owner ='SCOTT' 4 and table_name ='TEST';INDEX_NAME INDEX_TYPE UNIQUENES------------------------------ --------------------------- ---------IDX_TEST_ID NORMAL UNIQUE SQL>select constraint_name, constraint_type 2 from user_constraints 3 where table_name ='TEST';CONSTRAINT_NAME C------------------------------ -UK_TEST U SQL>alter table test disable constraint uk_test;Table altered. SQL>select constraint_name, constraint_type, status 2 from user_constraints 3 where table_name ='TEST';CONSTRAINT_NAME C STATUS------------------------------ - --------UK_TEST U DISABLED SQL>select index_name, index_type, uniqueness, status 2 from user_indexes 3 where table_owner ='SCOTT' 4 and table_name ='TEST';INDEX_NAME INDEX_TYPE UNIQUENES STATUS------------------------------ --------------------------- --------- --------IDX_TEST_ID NORMAL UNIQUE VALID 实验结果表明，先创建的唯一索引不受约束失效的影响。 SCOTT@ orcl>create index idx_test_idon test(id);Index created. SCOTT@ orcl>alter table testadd constraint uk_testunique (id);Table altered. SCOTT@ orcl>insert into testvalues(1,'liu');insert into testvalues(1,'liu')*ERRORat line 1:ORA-00001:unique constraint (SCOTT.UK_TEST) violated SCOTT@ orcl>alter table testdrop constraint uk_test;Table altered. SCOTT@ orcl>insert into testvalues(1,'liu');1 row created. SCOTT@ orcl>delete from testwhere id=1and rownum=1;1 row deleted. SCOTT@ orcl>select *from test; IDNAME---------- -------------------- yang 1 liu SCOTT@ orcl>create unique index idx_test_idon test(id);Index created. SCOTT@ orcl>alter table testadd constraint uk_testunique (id);Table altered. SCOTT@ orcl>alter table testdrop constraint uk_test;Table altered. SCOTT@ orcl>insert into testvalues(1,'liu');insert into testvalues(1,'liu')*ERRORat line 1:ORA-00001:unique constraint (SCOTT.IDX_TEST_ID) violated |
总结如下：（1）主键约束和唯一键约束均会隐式创建同名的唯一索引，当主键约束或者唯一键约束失效时，隐式创建的唯一索引会被删除；（2）主键约束要求列值非空，而唯一键约束和唯一索引不要求列值非空；（3）相同字段序列不允许重复创建索引；