Create README.md

Author: chenboy

src/optimizer/README.md

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+## Optimizer Overview
+
+The optimizer follows the cascade framework, it takes an annotated parse tree as input, goes through multiple optimization phases, and output an execution plan. The major phases are:
+
+* Parse tree to logical operator tree transformation, which turns a parse tree into a logical operator tree.
+* Predicate push-down, which pushes predicates to the lowest possible operator to evaluate.
+* Unnesting, which turns sub-queries with corelation into regular join operator.
+* Logical transformation, which enumerates all possible join order.
+* Stats derivation, which derives stats needed to compute cost for each group.
+* Phyisical implementation, which enumerates all possible implementation for a logical operator, e.g. hash join v.s. nested-loop join
+* Property enforcing, which adds missing properties descirbing the output data's format, e.g. sort order.
+* Operator to plan transformation, which turns the best physical operator tree into an execution plan.
+
+Predicate push-down and unnesting are rewrite phases, they will run independently.