Indexing Strategies:Leading Portion of an Index

Consider an index on columns a, b and c on table xyz.

Index is used for:

SELECT * FROM XYZ

WHERE a = 1

AND b = 2

AND c = 3

SELECT * FROM XYZ

WHERE a = 1

AND b = 2

SELECT * FROM XYZ

WHERE a = 1

ORDER BY a, b, c

Index is not used for:

SELECT * FROM XYZ

WHERE b = 2

AND c = 3

SELECT * FROM XYZ

WHERE b = 2

SELECT * FROM XYZ

WHERE c = 3

ORDER BY b, c