Sql not picking up primary key index

Hi,

I have the following table:

   SQL> desc conferencebookings;

 Name                                      Null?    Type
 ----------------------------------------- -------- --------------
 F_BOOKINGID                               NOT NULL NUMBER(10)
 F_CONFERENCEDETID                         NOT NULL NUMBER(10)
 F_PATTERNID                               NOT NULL NUMBER(10)
 F_BOOKERTYPE                              NOT NULL NUMBER(2)
 F_BOOKER                                  NOT NULL NUMBER(10)
 F_SOURCEIP                                         VARCHAR2(32)
 F_ROOMNUMBER                              NOT NULL NUMBER(9)
 F_AUTOEXTEND                              NOT NULL NUMBER(2)
 F_OWNERID                                          NUMBER(10)
 F_DTCREATED                               NOT NULL DATE
 F_DTSAVED                                 NOT NULL DATE

f_bookingid is the index.

I run the following sql:

SELECT F_ROOMNUMBER
FROM CONFERENCEBOOKINGS B,CONFERENCES C WHERE B.F_BOOKINGID = C.F_BOOKINGID explain plan is:

   SELECT STATEMENT Optimizer=FIRST_ROWS (Cost=91 Card=90 Bytes=1080)   NESTED LOOPS (Cost=91 Card=90 Bytes=1080)     TABLE ACCESS (FULL) OF CONFERENCEBOOKINGS (Cost=1 Card=90 Bytes=720)     INDEX (RANGE SCAN) OF CF_I3 (NON-UNIQUE) (Cost=1 Card=90 Bytes=360) It uses full table scan, where I would expect it to use the index on the pk.

Now if I change the select to :
  SELECT B.F_BOOKINGID
FROM CONFERENCEBOOKINGS B,CONFERENCES C WHERE B.F_BOOKINGID = C.F_BOOKINGID The explain plan now picks up the pk index:

   SELECT STATEMENT Optimizer=FIRST_ROWS (Cost=26 Card=90 Bytes=720)   NESTED LOOPS (Cost=26 Card=90 Bytes=720)     INDEX (FULL SCAN) OF CF_I3 (NON-UNIQUE) (Cost=26 Card=90 Bytes=360)     INDEX (UNIQUE SCAN) OF CFB_PK (UNIQUE) Any ideas ?? Received on Tue Sep 11 2001 - 06:40:14 CDT