Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for modular query optimizer. In one aspect, a method includes selecting one or more projections from a set of projections for each table in a database query wherein each of the selected projections for the table has leads to an estimated lower execution cost for the query as compared to non-selected projections; generating join orders for the query based on data distribution of one or more of the selected projections among sites in a computer network wherein the join orders reflect different combinations of data distribution operations applied to the output of one or more of the query's joins; and selecting a join order from the join orders based on evaluation of the join orders using a cost model.