Subquery processing and transformations

Subqueries are notoriously expensive to evaluate. This section describes some of the transformations that Derby makes internally to reduce the cost of evaluating them.

• Materialization
  Materialization means that a subquery is evaluated only once. There are several types of subqueries that can be materialized.
• Flattening a subquery into a normal join
  
• Flattening a subquery into an EXISTS join
  
• Flattening VALUES subqueries
  
• DISTINCT elimination in IN, ANY, and EXISTS subqueries
  
• IN/ANY subquery transformation