Query on a table with indexed date field

Question

I have a table with a date column which is indexed. If I run a query like "select column1 where date_field='20-JAN-04' for example it is fast and uses index.
If I run select column1 where date < '20-JAN-04' it is slow and doesnt use the index. I logged a TAR and Oracle told me that this is to be expected as not using the index in this case is the most effiecient way of doing the query.
Now my concept of an index is like the index of Yellow Pages(telephone directory) for example. In this example if I look for a name that is say "Halfords" or below, I can see all entries for Halfords and all the way to ZZZ in one block.
I just cant see , in a common sense way why Oracle wont use the index in this type of query.
George

Answer 1

Using the concept of a telephone directory is wrong. In a telephone directory you have all information order by the name. However in your table (if it is not an IOT) you don't have all information/rows ordered by your date_field. Rather think at the document "Oracle9i Database Concepts" and it's index.
Let's say you want to find all indexed words larger then "ISO SQL standard" (ok that doesn't make sense but it is just an example). So would it be faster to read the whole document or to lookup each word in the index and then read the entire page (Oracle block) to find the word.
It's not allways easy to know in advance if the query will be faster over the index or a full table scan. what you need to do is to well analyze (dbms_stats) the table and it's index, in most cases Oracle chooses the right way. You may also use the hint /*+ index(table_name index_name) */ and will see if it would be faster over the index or not.
A good document about that subject is:
http://www.ioug.org/tech/IOUGinDefense.pdf
HTH
Maurice