| ProjectionPruner |
The ProjectionPruner module is responsible for eliminating unnecessary column references (and other expressions) from the query. Projection pruning logically operates in two passes - the first pass is a top-down pass where information about all referenced columns and expressions is collected (pushed down from a node to its children). The second phase is a bottom-up phase, where each node (in response to the information collected above) attempts to rid itself of unwanted columns and expressions. The two phases can be combined into a single tree walk, where for each node, the processing is on the lines of: - compute and push information to children (top-down) - process children - eliminate unnecessary references from myself (bottom-up) |