Horrible advice you’ll sometimes encounter in your life as a database developer is that a DBA is responsible for indexes. Absolutely not! The performance of a SELECT depends on indexes, and the existence of indexes on a table affects INSERT, UPDATE, and DELETE statements. Only a developer knows what queries are typically run, how often a table’s data is modified, how it is modified (i.e. single rows or in bulk, normal or direct-path inserts, …) so only a developer can judge whether an index on a particular combination of columns makes sense.