I am using dbms_xplan.display to generate explain on a 9.2
database(see below). Just wondering what is what is the unit of the
last column "Cost" and "%CPU"? And why is the "Rows" show 12M where
an index is being used? Thanks.


SQL> explain plan for select * from seq where id = 1000000;
Explained.

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

--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |
Cost (%CPU)|
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 695 |
4 (25)|
| 1 | TABLE ACCESS BY INDEX ROWID| SEQ | 1 | 695 |
4 (25)|
|* 2 | INDEX UNIQUE SCAN | PK_SEQ | 12M| |
3 (34)|
--------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - access("SEQ"."ID"=1000000)

The cardinality of INDEX UNIQUE SCAN shows the full cardinality of the
table.
And the cardinality of TABLE ACCESS BY INDEX ROWID that follows it
shows as 1.

Whereas if you have an INDEX RANGE SCAN, the cardinality shows the
number of rows that would be returned. And the TABLE ACCESS BY INDEX
ROWID that follows it shows the same cardinality (provided there are
no additional predicates on the other table columns).

The cost is normally the number of I/O calls issued.
regards
Srivenu