index+unique constraint 和 unique index 的區別好像執行計劃不同~

SQL> drop table test;

Table dropped

SQL> create table test as select * from all_objects;

Table created

SQL> create index ind_TEST on test(object_id);

Index created

SQL> alter table test add constraint un_test unique(object_id) using index ind_test;

Table altered

SQL> insert into test select * from all_objects;

insert into test select * from all_objects

ORA-00001: 違反唯一約束條件 (SCOTT.UN_TEST)

SQL> explain plan for

2??select object_id from test where object_id=32;

Explained

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

Plan hash value: 2388774261

-----------------------------------------------------------------------------

| Id??| Operation? ?? ???| Name? ???| Rows??| Bytes | Cost (%CPU)| Time? ???|

-----------------------------------------------------------------------------

|? ?0 | SELECT STATEMENT |? ?? ?? ? |? ???1 |? ? 13 |? ???1? ?(0)| 00:00:01 |

|*??1 |??INDEX RANGE SCAN| IND_TEST |? ???1 |? ? 13 |? ???1? ?(0)| 00:00:01 |

-----------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - access("OBJECT_ID"=32)

13 rows selected

SQL> alter table TEST

2? ? drop constraint UN_TEST cascade;

Table altered

SQL> drop index ind_test;

Index dropped

SQL> create unique index un_ind_test on test(object_id);

Index created

SQL> insert into test select * from all_objects;

insert into test select * from all_objects

ORA-00001: 違反唯一約束條件 (SCOTT.UN_IND_TEST)

SQL> explain plan for

2??select object_id from test where object_id=32;

Explained

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

Plan hash value: 3033676969

--------------------------------------------------------------------------------

| Id??| Operation? ?? ?? ?| Name? ?? ???| Rows??| Bytes | Cost (%CPU)| Time

--------------------------------------------------------------------------------

|? ?0 | SELECT STATEMENT??|? ?? ?? ?? ? |? ???1 |? ? 13 |? ???1? ?(0)| 00:00:01

|*??1 |??INDEX UNIQUE SCAN| UN_IND_TEST |? ???1 |? ? 13 |? ???1? ?(0)| 00:00:01

--------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - access("OBJECT_ID"=32)

總結

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