In `aggregate/utils.scala`, there is a substantial amount of duplication in the expression-rewriting logic. As a prerequisite to supporting imperative aggregate functions in `TungstenAggregate`, this patch refactors this file so that the same expression-rewriting logic is used for both `SortAggregate` and `TungstenAggregate`.
In order to allow both operators to use the same rewriting logic, `TungstenAggregationIterator. generateResultProjection()` has been updated so that it first evaluates all declarative aggregate functions' `evaluateExpression`s and writes the results into a temporary buffer, and then uses this temporary buffer and the grouping expressions to evaluate the final resultExpressions. This matches the logic in SortAggregateIterator, where this two-pass approach is necessary in order to support imperative aggregates. If this change turns out to cause performance regressions, then we can look into re-implementing the single-pass evaluation in a cleaner way as part of a followup patch.
Since the rewriting logic is now shared across both operators, this patch also extracts that logic and places it in `SparkStrategies`. This makes the rewriting logic a bit easier to follow, I think.
Author: Josh Rosen <joshrosen@databricks.com>
Closes#9015 from JoshRosen/SPARK-10988.
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