Calculate size of data to be purged

This is going to be a difficult if not impossible. 

The delete vectors have only the epoch and position of the record to be deleted but the purge will remove the rows from the table. 

You could make a caluclation using the storage_containers table that has the deleted_rows, used_bytes and total_row_count and get what is the proporcion of deleted rows in the ros and see how much is taht in bytes. But this won't be an exact number of how much data will be purged because of the encoding, for example if you have a RLE column, even if you delete records, that won't change the used bytes because it will just change the count of the value, but won't reduce rows. Does make sense? 

Hope this helps. 

Eugenia