When constructing a selection_with_processing, split the
selectors into an inner loop and an outer loop with split_aggregation().
We can then reimplement add_input_row() and get_output_row() as follows:
- add_input_row(): evaluate the inner loop expressions and store
the results in temporaries
- get_output_row(): evaluate the outer loop expressions, pulling in
values from those temporaries.
reset(), which is called between groups, simply copies the initial
values rathered by split_aggregation() into the temporaries.
The only complexity comes from add_column_for_post_query_processing(),
which essentially re-does the work of split_aggregation(). It would
be much better if we added the column before split_aggregation() was
called, but some refactoring has to take place before that happens.
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