Function Based Index - Query Performance

Question

HI,
Good Day to All..
I'd like to use function based indexes on following column(to_char(ps.user_pc_id)).
Whereas this column is part of PRIMARY KEY.
Is it possible to create a function based index on PRIMARY KEY Column?
Attached below is the query with the explain plan ...
TO_CHAR Expression - Performance
Thanks for your reply.

Answer 1

DTYLER_APP@pssdev2> create table dt_fbi_pk(id varchar2(20));
Table created.
DTYLER_APP@pssdev2> drop table dt_fbi_pk;
Table dropped.
DTYLER_APP@pssdev2> create table dt_fbi(id number);
Table created.
DTYLER_APP@pssdev2> create index dt_fbi_idx on dt_fbi(to_char(id));
Index created.
DTYLER_APP@pssdev2> alter table dt_fbi add constraint dt_fbi_pk primary key (id);
Table altered.
DTYLER_APP@pssdev2> select constraint_name,constraint_type, index_name from user_constraints where table_name='DT_FBI';
CONSTRAINT_NAME                C INDEX_NAME
DT_FBI_PK                      P DT_FBI_PK
1 row selected.When we created the primary key constraint, Oracle created a new index rather than using the existing one because....
DTYLER_APP@pssdev2> alter table dt_fbi drop primary key;
Table altered.
DTYLER_APP@pssdev2> select index_name from user_indexes where table_name ='DT_FBI';
INDEX_NAME
DT_FBI_IDX
1 row selected.
DTYLER_APP@pssdev2> alter table dt_fbi add constraint dt_fbi_pk primary key (id) using index dt_fbi_idx;
alter table dt_fbi add constraint dt_fbi_pk primary key (id) using index dt_fbi_idx
ERROR at line 1:
ORA-14196: Specified index cannot be used to enforce the constraint.
DTYLER_APP@pssdev2> alter table dt_fbi add constraint dt_fbi_uk unique(id) using index dt_fbi_idx;
alter table dt_fbi add constraint dt_fbi_uk unique(id) using index dt_fbi_idx
ERROR at line 1:
ORA-14196: Specified index cannot be used to enforce the constraint.We can't use a function based index to enforce a unique or primary key constraint. Changing the syntax does not help..
DTYLER_APP@pssdev2> alter table dt_fbi add constraint dt_fbi_uk unique(TO_CHAR(id)) using index dt_fbi_idx;
alter table dt_fbi add constraint dt_fbi_uk unique(TO_CHAR(id)) using index dt_fbi_idx
ERROR at line 1:
ORA-00904: : invalid identifierWe can create a unique index however
DTYLER_APP@pssdev2> drop index dt_fbi_idx;
Index dropped.
DTYLER_APP@pssdev2> create unique index dt_fbi_idx on dt_fbi(to_char(id));
Index created.but we still can't use it to enforce a unique or primary key constraint
DTYLER_APP@pssdev2> alter table dt_fbi add constraint dt_fbi_pk primary key (id) using index dt_fbi_idx;
alter table dt_fbi add constraint dt_fbi_pk primary key (id) using index dt_fbi_idx
ERROR at line 1:
ORA-14196: Specified index cannot be used to enforce the constraint.
DTYLER_APP@pssdev2> alter table dt_fbi add constraint dt_fbi_uk unique(id) using index dt_fbi_idx;
alter table dt_fbi add constraint dt_fbi_uk unique(id) using index dt_fbi_idx
ERROR at line 1:
ORA-14196: Specified index cannot be used to enforce the constraint.So no, you can't use it for a primary key. If you just want to enforce uniqueness then yes, you can do it with a unique index, but not a constraint.
DTYLER_APP@pssdev2> select * from v$version;
BANNER
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE    10.2.0.4.0      Production
TNS for Linux: Version 10.2.0.4.0 - Production
NLSRTL Version 10.2.0.4.0 - Production
5 rows selected.HTH
Daviid