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,
  name varchar2(20),
  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
  from user_indexes
  where table_owner='SCOTT'
  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,
  name varchar2(20),
  constraint uk_test unique(id));
Table created.
SQL> select constraint_name, constraint_type from user_constraints;
CONSTRAINT_NAME                C
------------------------------ -
UK_TEST                        U

 

查看此时的索引情况:
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SQL> select index_name, index_type, uniqueness, tablespace_name
  from user_indexes
  where table_owner='SCOTT'
  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,
  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,
  name varchar2(20),
  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

 

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,
  name varchar2(20));
Table created.
SQL> create unique index idx_test_id on test (id);
Index created.
SQL> select index_name, index_type, uniqueness
  from user_indexes
  where table_owner = 'SCOTT'
  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
  from user_indexes
  where table_owner = 'SCOTT'
  and table_name = 'TEST';
INDEX_NAME                     INDEX_TYPE                  UNIQUENES
------------------------------ --------------------------- ---------
IDX_TEST_ID                    NORMAL                      UNIQUE
SQL> select constraint_name, constraint_type
  from user_constraints
  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
  from user_constraints
  where table_name = 'TEST';
CONSTRAINT_NAME                C STATUS
------------------------------ - --------
UK_TEST                        U DISABLED
SQL> select index_name, index_type, uniqueness, status
  from user_indexes
  where table_owner = 'SCOTT'
  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;
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