Heap Structures
Heaps have one row in sysindexes with indid = 0. The column sysindexes.FirstIAM points to the first IAM page in the chain of IAM pages that manage the space allocated to the heap. Microsoft® SQL Server™ 2000 uses the IAM pages to navigate through the heap. The data pages and the rows within them are not in any specific order, and are not linked together. The only logical connection between data pages is that recorded in the IAM pages.
Table scans or serial reads of a heap can be done by scanning the IAM pages to find the extents holding pages for the heap. Because the IAM represents extents in the same order they exist in the data files, this means that serial heap scans progress uniformly down each file. This is more efficient than following the data page chains used in earlier versions of SQL Server, in which the data page chain often takes a somewhat random path through the files of a database. Using the IAM pages to set the scan sequence also means that rows from the heap are not typically returned in the order in which they were inserted.
