Understanding the COST column of an explain plan

Question

Hello,
I executed the following query, and obtained the corresponding explain plan:
select * from isis.clas_rost where cour_off_# = 28
Description COST Cardinality Bytes
SELECT STATEMENT, GOAL = FIRST_ROWS               2     10     1540
TABLE ACCESS BY INDEX ROWID     ISIS     CLAS_ROST     2     10     1540
INDEX RANGE SCAN     ISIS     CLAS_ROST_N2     1 10     
I don't understand how these cost values add up. What is the significance of the cost in each row of the explain plan output?
By comparison, here is another plan output for the following query:
select * from isis.clas_rost where clas_rost_# = 28
Description COST Cardinality Bytes
SELECT STATEMENT, GOAL = FIRST_ROWS               1     1     154
TABLE ACCESS BY INDEX ROWID     ISIS     CLAS_ROST     1     1     154
INDEX UNIQUE SCAN     ISIS     CLAS_ROST_U1     1 1     
Thanks!

Answer 1

For the most part, you probably want to ignore the cost column. The cardinality column is generally what you want to pay attention to.
Ideally, the cost column is Oracle's estimate of the amount of work that will be required to execute a query. It is a unitless value that attempts to combine the cost of I/O and CPU (depending on the Oracle version and whether CPU costing is enabled) and to scale physical and logical I/O appropriately). As a unitless number, it doesn't really relate to something "real" like the expected number of buffer gets. It is also determined in part by initialization parameters,session settings, system statistics, etc. that may artificially increase or decrease the cost of certain operations.
Beyond that, however, cost is problematic because it is only as accurate as the optimizer's estimates. If the optimizer's estimates are accurate, that implies that the cost is reasonably representative (in the sense that a query with a cost of 200 will run in less time than a query with a cost of 20000). But if you're looking at a query plan, it's generally because you believe there may be a problem which means that you are inherently suspicious that some of the optimizer's estimates are incorrect. If that's the case, you should generally distrust the cost.
Justin