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Multi-column Tree Indexes

Why can some predicates on (A, B, C) be evaluated using an index, but not others?

Suppose we have this index (showing just key columns: A, B, C).

Index keys

A B C
2 1 3
2 1 6
2 1 8
2 5 5
2 7 3
2 7 9
3 5 4
3 5 6
3 8 1
3 8 7
3 8 9

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