Which statement best describes composite indexes?

• A. An index on multiple columns to support queries with multiple criteria.
• B. An index on a single column.
• C. An index that slows reads.
• D. An index automatically created for all tables.

Answer: (A)

Composite indexes are indexes built on multiple columns. They let the database quickly satisfy queries that specify conditions on several columns, because the index is organized by the combined values of those columns. This speeds up searches when you filter on more than one field at a time and can also support sorts or groupings that align with the index order. The order of the columns in the index matters: an index on (A, B) helps with filtering on A (and on A plus B) more than with filtering on B alone. This type of index is not the same as a single-column index, and it doesn’t slow reads; it generally speeds them by reducing the number of rows the engine has to scan. It’s also not automatically created for every table—you define it explicitly. That’s why describing composite indexes as an index on multiple columns to support queries with multiple criteria is the best explanation.