Fix mis-pruned data files

[dentiny]

Closes #1135

The bug is: when we get data files, current implementation only select those with columns which matches the predicate (i.e., col_b <= 42), but miss all data files without the requested column.

Data file query before this fix:

data.data_file_id IN (
	SELECT data_file_id FROM col_2_stats
	WHERE (value_count IS NULL OR value_count > 0)
	  AND (min_value IS NULL OR max_value IS NULL OR (min_value <= '42' AND max_value >= '42'))
  )

After the fix

(
	data.data_file_id NOT IN (SELECT data_file_id FROM col_2_stats)
	OR
	data.data_file_id IN (
	  SELECT data_file_id FROM col_2_stats
	  WHERE (value_count IS NULL OR value_count > 0)
		AND (min_value IS NULL OR max_value IS NULL OR (min_value <= '42' AND max_value >= '42'))
	)
  )