What are the responsibilities of a DBA?

The DBA is responsible for:

• Designing the logical and physical schemas, as well as widely-used portions of the external schema. 
• Security and authorization. 
• Data availability and recovery from failures. 
• Database tuning: The DBA is responsible for evolving the database, in particular the conceptual and physical schemas, to ensure adequate performance as user requirements change.

A DBA needs to understand query optimization even if s/he is not interested in running his or her own queries because some of these responsibilities (database design and tuning) are related to query optimization. Unless the DBA understands the performance needs of widely used queries, and how the DBMS will optimize and execute these queries, good design and tuning decisions cannot be made.