Highlights of System R Optimizer
Impact:
- Most widely used currently; works well for < 10 joins.
Cost estimation: Approximate art at best.
- Statistics, maintained in system catalogs, used to estimate cost of operations and result sizes.
- Considers combination of CPU and I/O costs.
Plan Space: Too large, must be pruned.
- Only the space of left-deep plans is considered.
- Left-deep plans allow output of each operator to be pipelined into the next operator without storing it in a temporary relation.
- Cartesian products avoided when possible.