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5 The MAL Optimizer
One of the prime reasons to design the MAL intermediate language is to have a high-level description for database queries, which is easy to generate by a front-end compiler and easy to decode, optimize and interpret.
An optimizer needs several mechanisms to be effective. It should be able to perform a symbolic evaluation of a code fragment and collect the result in properties for further decision making. The prototypical case is where an optimizer estimates the result size of a selection.
Another major issue is to be able to generate and explore a space of alternative evaluation plans. This exploration may take place up front, but can also be ran at runtime for query fragments.