Oracle主键、唯一键与唯一索引的区别
一般，我们看到术语“索引”和“键”交换使用，但实际上这两个是不同的。索引是存储在
数据库中的一个物理结构，键纯粹是一个逻辑概念。键代表创建来实施业务规则的完整性约束。索引和键的混淆通常是由于数据库使用索引来实施完整性约束。
接下来我们看看数据库中的主键约束、唯一键约束和唯一索引的区别。
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SQL>
select
*
from
v$version;
BANNER
--------------------------------------------------------------------------------
Oracle
Database
11g Enterprise Edition Release 11.2.0.1.0 - 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>
create
table
test (
2 id
int
,
3
name
varchar2(20),
4
constraint
pk_test
primary
key
(id))
5 tablespace users;
Table
created.
SQL>
select
constraint_name, constraint_type
from
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：
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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：
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SQL>
create
unique
index
idx_test_uk
on
test(id);
create
unique
index
idx_test_uk
on
test(id)
*
ERROR
at
line 1:
ORA-01408: such
column
list already indexed
|
即使创建非唯一索引也不行：
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SQL>
create
index
idx_test_id
on
test(id);
create
index
idx_test_id
on
test(id)
*
ERROR
at
line 1:
ORA-01408: such
column
list already indexed
|
那么唯一键约束的情况是怎样的呢？
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SQL>
drop
table
test purge;
Table
dropped.
SQL>
create
table
test(
2 id
int
,
3
name
varchar2(20),
4
constraint
uk_test
unique
(id));
Table
created.
SQL>
select
constraint_name, constraint_type
from
user_constraints;
CONSTRAINT_NAME C
------------------------------ -
UK_TEST U
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查看此时的索引情况：
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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？
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SQL>
insert
into
test
values
(1,
'Sally'
);
1 row created.
SQL>
insert
into
test
values
(
null
,
'Tony'
);
1 row created.
SQL>
insert
into
test
values
(
null
,
'Jack'
);
1 row created.
SQL>
select
*
from
test;
ID
NAME
---------- --------------------
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。
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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_id
on
test (id);
Index
created.
SQL>
insert
into
test
values
(1,
'Sally'
);
1 row created.
SQL>
insert
into
test
values
(
null
,
'Tony'
);
1 row created.
SQL>
insert
into
test
values
(
null
,
'Jack'
);
1 row created.
SQL>
select
*
from
test;
ID
NAME
---------- --------------------
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？
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SQL>
drop
table
test purge;
Table
dropped.
SQL>
create
table
test(
2 id
int
,
3
name
varchar2(20),
4
constraint
uk_test
unique
(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
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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 Uniqueness
You 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？
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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_id
on
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
test
add
constraint
uk_test
unique
(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_id
on
test(id);
Index
created.
SCOTT@ orcl>
alter
table
test
add
constraint
uk_test
unique
(id);
Table
altered.
SCOTT@ orcl>
insert
into
test
values
(1,
'liu'
);
insert
into
test
values
(1,
'liu'
)
*
ERROR
at
line 1:
ORA-00001:
unique
constraint
(SCOTT.UK_TEST) violated
SCOTT@ orcl>
alter
table
test
drop
constraint
uk_test;
Table
altered.
SCOTT@ orcl>
insert
into
test
values
(1,
'liu'
);
1 row created.
SCOTT@ orcl>
delete
from
test
where
id=1
and
rownum=1;
1 row deleted.
SCOTT@ orcl>
select
*
from
test;
ID
NAME
---------- --------------------
yang
1 liu
SCOTT@ orcl>
create
unique
index
idx_test_id
on
test(id);
Index
created.
SCOTT@ orcl>
alter
table
test
add
constraint
uk_test
unique
(id);
Table
altered.
SCOTT@ orcl>
alter
table
test
drop
constraint
uk_test;
Table
altered.
SCOTT@ orcl>
insert
into
test
values
(1,
'liu'
);
insert
into
test
values
(1,
'liu'
)
*
ERROR
at
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；