Sequential access

When you use a sequential access method, the segments in the database are stored in hierarchic sequence, one after another, with no pointers.

IMS full-function has two sequential access methods. Like the direct access methods, one has an index and the other does not:

• HSAM only processes root segments and dependent segments sequentially.
• HISAM processes data sequentially but has an index so that you can access records directly. HISAM is primarily for sequentially processing dependents, and directly processing database records.

Some of the general requirements that sequential access satisfies are:

• Fast sequential processing
• Direct processing of database records with HISAM
• Small IMS overhead on storage because sequential access methods relate segments by adjacency rather than with pointers

The three disadvantages of using sequential access methods are:

• Sequential access methods give slower access to the right-most segments in the hierarchy, because HSAM and HISAM must read through all other segments to get to them.
• HISAM requires frequent reorganization to reclaim space from deleted segments and to keep the logical records of a database record physically adjoined.
• You cannot update HSAM databases. You must create a new database to change any of the data.