Question

Suppose you have a table with 2 fields, like first name and last name.

If you add a composite index on both fields, that should theoretically take care of indexing the first field. Creating a second index on that field is redundant, as far as I know. To make sure all queries use indexes, you would only have to add an index to last name.

So it seems that the number of indexes on a table should not be larger than the number of fields in a table.

Is that correct or not? If not, why not?

Answer 1

Consider a table T with 3 index-worthy columns: A, B, C.

1. The primary index might be on T(A, B, C).
2. There might be queries for which A and C are defined, so the index for those is T(A, C).
3. There might be queries for which B is defined: T(B) is the index.
4. There might be queries for which C is defined: T(C) is the index.

That looks like more indexes than columns.

The more columns there are in the table, the easier it is to come up with possible indexes that might help — and might push the index count higher than the column count.