6. The method of claim 1 , further comprising: determining that the particular set operator comprises a condition that is satisfied for a set of values if the condition is satisfied individually by any values of the set, wherein the condition applies differently to null values than to non-null values; based at least in part on determining that the particular set operator comprises the condition, eliminating the particular subquery and the particular set operator, and generating the added predicate such that the added predicate applies differently to null values than to non-null values.
6. The method of claim 1 , further comprising: determining that the particular set operator comprises a condition that is satisfied for a set of values if the condition is satisfied individually by any values of the set, wherein the condition applies differently to null values than to non-null values; based at least in part on determining that the particular set operator comprises the condition, eliminating the particular subquery and the particular set operator, and generating the added predicate such that the added predicate applies differently to null values than to non-null values. 7. The method of claim 6 , wherein the condition is defined at least in part by at least one of an EXISTS operator or an ANY operator.