Multiple index scenario

Question

Suppose I have a table that has (among other columns) the following columns:
LAST_UPDATE DATE,
CATEGORY VARCHAR(1) NOT NULL,
The statistics of the data are as follows:
- CATEGORY can only be one of 5 different values: 'A', 'B', 'C', 'D' and 'E'
- The distribution of the DATE values are pretty uniform. For instance, the number of rows in any 10-minute period is pretty much the same as any other 10-minute period.
- The number of rows for a given value of LAST_UPDATE is almost always 0 or 1. In other words, SELECT * ... WHERE LAST_UPDATE = :p1 almost never returns more than one row.
- The distribution of the CATEGORY values are roughly equally proportioned.
- There are a lot of rows in the table
I want to optimize the following queries:
A. SELECT ... FROM ... WHERE LAST_UPDATE BETWEEN :p1 and :p2;
B. SELECT ... FROM ... WHERE LAST_UPDATE BETWEEN :p1 and :p2 and CATEGORY = :p3;
Even though it generally is a mistake to ask two questions in the same forum post, here I go:
1. Will an index on (LAST_UPDATE) work better/worse/the same than one on (LAST_UPDATE, CATEGORY)?
2. Will Oracle be able to use an index on (CATEGORY, LAST_UPDATE) for the first query?
My intuition tells me that an index on (LAST_UPDATE) alone should just work as well as the multi-column index (LAST_UPDATE, CATEGORY). In fact, the multi-column index might be less performant in that it might take up more space and make Oracle do more unhelpful work.
As for #2, my understanding is that Oracle can use an index on (CATEGORY, LAST_UPDATE) for query A. It will effectively rewrite the query as a UNION:
SELECT ... FROM WHERE (LAST_UPDATE BETWEEN :p1 and :p2) AND (CATEGORY = 'A')
UNION_ALL
SELECT ... FROM WHERE (LAST_UPDATE BETWEEN :p1 and :p2) AND (CATEGORY = 'B')
UNION_ALL
SELECT ... FROM WHERE (LAST_UPDATE BETWEEN :p1 and :p2) AND (CATEGORY = 'E')
Comments?
Edited by: user10936714 on Nov 21, 2011 12:39 PM
Edited by: user10936714 on Nov 21, 2011 12:47 PM

Answer 1

My intuition tells me that...Why not put together a test scenario?