System R Optimizer  Developed at IBM starting in the 1970's  Most widely used currently; works well for up to 10 joins  Cost estimation  Statistics maintained in system catalogs  Used to estimate cost of operations and result sizes  Query Plan Space  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
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