What is Bookmark Lookup?
If all the columns required by the query are not available in the nonclustered index itself, a lookup is required to base table to pick those columns which are not part of the nonclustered index being used to retrieve the data. Lookup fetches the corresponding data row from the table by following the row locator value from the index row. It requires extra logical read on data page excluding the logical read on the index page.
Types of lookup
This lookup may be a RID lookup against a clustered index or a key lookup against a heap.
If base table does not have any clustered index created on it, a Row ID of heap, will be used as a row locator and if any query which will require a lookup using this heap row id row locator will perform a lookup, the lookup will … More