Special overflow pages are created for nonunique clustered indexes on allpages-locked tables when a newly inserted row has the same key as the last row on a full data page. A new data page is allocated and linked into the page chain, and the newly inserted row is placed on the new page (see Figure 9-4).
Figure 9-4: Adding an overflow page to a clustered index, allpages-locked table
The only rows that will be placed on this overflow page are additional rows with the same key value. In a nonunique clustered index with many duplicate key values, there can be numerous overflow pages for the same value.
The clustered index does not contain pointers directly to overflow pages. Instead, the next page pointers are used to follow the chain of overflow pages until a value is found that does not match the search value.