Ed Elliott takes us through things to think about before deleting a few million rows from a table:
Fragmentation
Fragmentation occurs when we delete from pages, and there is still data surrounding our data. If we have 100 rows and delete every odd row, we would have perfect fragmentation in that we have doubled the size of the data that we need. If we delete rows 1-49, even though we remove the same number of rows we don’t have any fragmentation as the data is in a continuous block. Knowing how the data is stored on disk and how the data will be deleted, is it the first x records or every x record is vital so that we know whether, after the delete, we should also reorganise the indexes to remove the deleted records.
Ed has quality insights here, so check it out.