Organization of Data

Let's start with the definition of a page. It is a physical unit on the disk. Data is stored in it. The size of a page in SQL Server is 8192 bytes (8kb). Each page belongs to a single table, index, or indexed view.

Next let's talk about a heap. It is made up of pages on disk. There is no ordering of data in the heap. Thus for tables stored in the heap, there is no clustered index on the table. Within the heap, rows are referenced via a row identifier (RID), which is an 8-byte value.

Now that we understand what a page and heap are, let's talk about the Index Allocation Map (IAM). This itself is stored as a system page. It points to space in the heap. The internal structure is a doubly linked list. It is used to scan through the heap. When you first insert data into a table, the IAM is created.