How to use indexes correctly in oracle

Question

Hi guys
on one table i have indexes on 3 columns in the following order
a
b
c
When i m writing a select statement in my where clause what should be the order? like
where
a='some value' and
b='some value' and
c='some value';
or in reverse order like this
c='some value' and
b='some value' and
a='some value';
please let me know.
Thanks

Answer 1

If you have an index on a,b,c the difference in performance can be on the index order.
If column "a" has only 2 unique values then the sub values of the index only has 50% of the remaining values, the cost optimizer on Oracle will probably skip the index as an index scan of 50% is slower than a full table scan.
If a has 100 unique values then the remaining search is only on 1% of the values so is likely to be used.
As with any optimisation try using
explain plan for selec x,y,z from a_table
from where
a='some value' and
b='some value' and
c='some value'
If the index is not being used firstly try
analyze table estimate statistics
sample 10 percent
for all indexes
for all indexed columns;
and failing than try different orders in acceptance environment.