What do I miss in understanding the clustered index?

Question

In absence of any index the table rows are accessed through IAM ((Index Allocation Map).
Can I directly access a row programmatically using IAM?

Does absen

Answer 1

That's a lot of questions. Yes the IAM is used to look up pages on a heap. The difference is that without an index there is no way to know what pages to retrieve for any given piece of data. An important feature of the SQL / relational model of data is that queries access data by data values only - never by using pointers or other structures directly.

A slot number just identifies a row within a page. Row data is not logically ordered within a page, even in a clustered index. Each data page contains a row offset table that points to the position of rows within a page.

A clustered index can slow down data access from nonclustered indexes because of the additional bookmark lookups required. This can be mitigated by using the INCLUDE clause to add columns to a NC index. Sometimes it may be more efficient not to have a clustered index on a table.