Unexpected SQL execution timing

Question

Oracle Version 9i
Win 2000/XP
We have one table comm which is a huge partitioned table (range - quarter). We had one time-taking statement that we were trying to tune -
DELETE FROM comm WHERE paid <> 'Y';
The explain plan told us that the index was not getting hit (which is understandable). The cost was around 270K odd.
We changed the statement to delete a similar numer of records -
DELETE FROM comm WHERE paid = 'N';
DELETE FROM comm WHERE paid = 'X';
DELETE FROM comm WHERE paid = 'ZZ';
This time the index was getting hit but the cost was still around 260K odd. However, the time taken to execute the 3 given statements went down from 40-minutes to 1minute!
What I am not able to understand is how come the Cost was so way off in estimation. Thanks in advance for your responses.
Additional info - the 3 statements will always use the latest partition only. However, the explain plan shows that it traverses all the partitions in both the scenarios.

Answer 1

The query itself does not hit a specific partition - you said that the PAID column is a local partitioned index. This means that the DELETE predicate does not allow for partition pruning - only hitting specific partitions.
So depending on the volume of data to process, this can be very slow. Which is the next point. The way the predicate is constructed. A "PAID <> value" is very different than "PAID = value1 OR PAID = value2". The first requires an index range scan (if the index can be used optimally) to determine which rows have indexed values that are not equal to the stated value.
Range scans can be problem - the bigger the range, the more I/O is needed. I/O is the slowest and most expensive of all RDBMS operations.
Compare this to telling the CBO the value of PAID that you're looking for - the ideal way an index should be used.
As for the CBO costs - ignore it. Comparing the cost of one query with that of another query (or even similar query) is not a sensible thing to do. To read up what experts say on it, go to http://asktom.oracle.com and do a search on "comparing cbo costs" or something along those lines. Lots of interesting stuff there.
Is PAID index a bitmap index? Or a standard B+tree index?