A good page split occurs when SQL Server inserts into a full final index page, allocates a new page at the end, and preserves existing row order without moving data. section 9.2 section 11.1
Page splits are normal, not automatically harmful. The important distinction is location: splitting the last page avoids the out-of-order pages and avoidable I/O associated with a middle-of-index split. section 11.1
A full index reaches its final page
An index is full and SQL Server must add a row to its last page. It allocates the next page from the extent and inserts the row there; existing rows remain in place and order is preserved. section 9.2 section 11.1
Figure 9.1 Good page splits
Use the figure to compare a good final-page allocation with a middle-of-index split. section 9.2
