A rule is used to modified a relational expression (ie query plan)
They are used by the planner to:
- or modify
relational algebra expressions (ie query plan).
- Converters rule implement Converter and convert from one convention to an another.
- Converter rules applied via convert
A ConverterRule converts between calling conventions.
- indicates what system will actually execute the query by assigning a particular call.
Example: exchange the order of Filter and a Project nodes.
Match: Transformer Rules are matched to elements of a query plan using pattern matching. onMatch is called for matched rules