Key Constraints in the Index Tables

Question

Hey,
I am a starter in the Database Application Development.
I have created few indexes on columns of different tables, and I wanted to look into the ALL_INDEXES and the USER_INDEXES to find the indexes that I created.
I noticed that there are some primary keys listed in the ALL_INDEXES or USER_INDEXES tables.
Can somebody tell me why the Primary Keys that are defined on tables are also getting listed in the USER_INDEXES or ALL_INDEXES tables.
Thanks a Million

Answer 1

That's very simple.
To be a primary key a column must verify two conditions:
1) is not nullable
2) is unique
Indexes can be declared unique, hence the simplest way oracle has to create a primary key constraint is to create an authomatuc unique index on it:
SQL>  create table x (a number);
Table created.
SQL> select * from user_indexes where table_name='X';
no rows selected
SQL> desc x;
Name                                                     Null?    Type
A                                                                 NUMBER
SQL> alter table x add constraint x_pk primary key (a);
Table altered.
SQL> desc x;
Name                                                     Null?    Type
A                                                        NOT NULL NUMBER
SQL> -- A has become NOT NULL
SQL> select index_name, uniqueness
  2   from user_indexes where table_name='X';
INDEX_NAME                     UNIQUENES
X_PK                           UNIQUE
SQL> -- an authomatic index has been created
SQL> drop index X_PK;
drop index X_PK
ERROR at line 1:
ORA-02429: cannot drop index used for enforcement of unique/primary key
SQL> -- You cannot drop it because it enforces the primary key
SQL> alter table x drop constraint x_pk;
Table altered.
SQL> -- but if you drop the primary key...
SQL> select index_name, uniqueness
  2   from user_indexes where table_name='X';
no rows selected
SQL> -- ...the index too is automatically dropped.Max
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