Composite indexes include multiple columns. Column order matters for query optimization.
The left-to-right rule:
Index on (a, b, c) can be used for:
WHERE a = 1WHERE a = 1 AND b = 2WHERE a = 1 AND b = 2 AND c = 3
Cannot be used efficiently for:
WHERE b = 2(skips first column)WHERE a = 1 AND c = 3(gap in middle)
Design principle: Put equality columns first, range columns last.