Identify the missing word(s) in the following sentence within the context of Microsoft Azure. SQL Server has several index types to support different types of workloads. At a high level, an index can be thought of as an on-disk structure that is associated with a table or a view, that enables SQL Server to more easily find the row or rows associated with the index key, compared to scanning the entire table. A(n) [?] is the underlying table, stored in sorted order based on the key value. There can only be one [?] on a given table, because the rows can be stored in one order.