Performing Group-By before Join

Assume that we have an SQL query containing joins and a group by The standard way of evaluating this type of query is to rst perform all the joins and then the group by operation However it may be possible to perform the group by early that is to push the group by operation past one or more joins Early grouping may reduce the query processing cost by reducing the amount of data participating in joins We formally de ne the problem adhering strictly to the semantics of NULL and dupli cate elimination in SQL and prove necessary and su cient conditions for deciding when this transformation is valid In practice it may be ex pensive or even impossible to test whether the conditions are satis ed Therefore we also present a more practical algorithm that tests a sim pler su cient condition This algorithm is fast and detects a large sub class of transformable queries Authors email addresses fpwyan palarsong bluebox uwaterloo ca