Select distinct is using the wrong index

Question

Hello,
I have an indexes question?
I have a table with 1M rows, the table structure is as follows:
create table X(aTime DATE , a1 NUMBER(28,0), a2 NUMBER(28,0), a3 NUMBER(28,0), .... , a20 NUMBER(28,0) );
I have also 2 indexes:
- IDX1 is defined on (aTime, a1, a2, a3).
- IDX2 is defined on (a1, a2, a3).
I run the following query: "SELECT DISTINCT A1, A2, A3 from X;" and see that the optimizer chooses to do a FTS (full table scan) though I was expecting him to use IDX2 because it looks like a "covering" indexes giving all the answers to the query without fetching data rows.
When I try to force him to use indexes, using the following hint "/*+INDEX(X IDX2)*/" the optimizer chooses IDX1.
When I use same hint after I dropped IDX1 the optimizer still prefers to make FTS .
Some general info:
- a1, a2 and a3 have about 500 distinct values each).
- db version 8.1.7.4.
- all tables and indexes are analyzed.
I have two questions:
===============
1.
Why does the optimizer prefers IDX1 (aTime, a1, a2, a3) over IDX2 (a1, a2, a3) when the query specifically requests distinct values of (aTime, a1, a2, a3), isn't there a shortcut the optimizer should use here?
2.
When I request the optimizer to use specific index (using hints), shouldn't it use the index I request regardless of his calculations? If not, is there any way to change this behavior?
Tal Olier
[email protected]

Answer 1

You said you analyzed the tables and indexes. Did you also analyze the indexed columns?
A full table scan may be a better execution plan than using an index, especially if you are selecting the full table.
Remember that a hint is jut that, a hint. It suggests to the optimizer to do something, but the optimizer may still choose a different plan.