When you create a composite B-tree index, you should consider the following.
The most restrctive condition: The condition that returns the least number of rows. The column that is involved in this condition, should be the leading column of B-tree index.
So for finding the candidate of composite index columns, column order should be..
( Column involved in the most restrictive condition, …….. , column involved in the least restrictive condition).
Also, consider the data distribution while checking restrictive columns. Above analysis will work better for uniformly distributed column values.