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Abstract : |
Efficient processing of aggregation queries is essential for decision support applications. This paper describes a class of query transformations, called eager aggregation and lazy aggregation, that allows a query optimizer to move group-by operations up and down the query tree. Eager aggregation partially pushes a group-by past a join. After a group-by is partially pushed down, we still need to perform the original group-by in the upper query block. Eager aggregation reduces the number of input rows to the join and thus may result in a better overall plan. The reverse transformation, lazy aggregation, pulls a group-by above a join and combines two group-by operations into one. This transformation is typically of interest when an aggregation query references a grouped view (a view containing a group-by). Experimental results show that the technique is very beneficial for queries in the TPC-D benchmark., |
