INDEX RANGE SCAN and INDEX UNIQUE SCAN

Had some time to dwell on INDEX RANGE SCAN and INDEX UNIQUE SCAN when the number of rows in a table are unique.

Consider,

create table temp as

select object_id from dba_objects

/

Now, we create a unique index on this table

SQL> create unique index id1 on temp(object_id) compute statistics

2 /

Index created.

Now, if I give

SQL> select 'x' from temp where object_id='5642'

2 /

Execution Plan

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

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=3)

1 0 INDEX (UNIQUE SCAN) OF 'ID1' (UNIQUE) (Cost=1 Card=1 Bytes

=3)

Statistics

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

0 recursive calls

0 db block gets

2 consistent gets

0 physical reads

0 redo size

211 bytes sent via SQL*Net to client

276 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

Now, If the unique index is dropped and a normal index is created, we get

SQL> create index id1 on temp(object_id) compute statistics

2 /

SQL> select 'x' from temp where object_id='5642'

2 /

Execution Plan

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

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=3)

1 0 INDEX (RANGE SCAN) OF 'ID1' (NON-UNIQUE) (Cost=1 Card=1 By

tes=3)

Statistics

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

0 recursive calls

0 db block gets

3 consistent gets

0 physical reads

0 redo size

211 bytes sent via SQL*Net to client

276 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

Why do we have an additional '1' consistent gets in this case?