1. A method implemented and programmed within a non-transitory computer-readable storage medium and processed by one or more processors, the processors configured to execute the method, comprising: detecting in a query an outer join on a join table and a second table, the query also including a GROUP BY clause; expanding the GROUP BY clause to double a total number of groups in a first level partition of the GROUP BY clause for the second table by generating additional query statements in an expanded query that when executed produce a first list of first attributes that satisfy an ON clause predicate for the second table on an ON clause and a second list of attributes that satisfy a join predicate on the ON clause for the second table; and producing a modified query having the expanded GROUP BY clause, the modified query is equivalent to the query.