Optimizer performs following operations.

• Evaluation of expressions & checks: Syntactic (Syntax of query), Semantic (objects exist and accessible)
• Statement transformation: Transform view and sub-quires into a  equivalent join / base tables
• Choice of optimizer goals: Chooses throughput or best response time.
• Choice of access paths: Chooses one or more available access paths
• Choice of join orders: Chooses which pair of tables is joined first