BC remote Oracle DBA - Call (800) 766-1884  
Oracle Consulting Oracle Training Development

Remote DBA

Remote DBA Plans  

Remote DBA Service

Remote DBA RAC

Remote DBA Oracle Home
Remote DBA Oracle Training
Remote DBA SQL Tuning Consulting
Remote DBA Oracle Tuning Consulting
Remote DBA Data Warehouse Consulting
Remote DBA Oracle Project Management
Remote DBA Oracle Security Assessment
Remote DBA Unix Consulting
Burleson Books
Burleson Articles
Burleson Web Courses
Burleson Qualifications
Oracle Links
Remote DBA Oracle Monitoring
Remote DBA Support Benefits
Remote DBA Plans & Prices
Our Automation Strategy
What We Monitor
Oracle Apps Support
Print Our Brochure
Contact Us (e-mail)
Oracle Job Opportunities
Oracle Consulting Prices





Remote DBA services

Remote DBA Support

Remote DBA RAC

Remote DBA Reasons

Remote Oracle Tuning

Remote DBA Links

Oracle DBA Support

Oracle DBA Forum

Oracle Disaster

Oracle Training

Oracle Tuning

Oracle Training

 Remote DBA SQL Server

Remote MSSQL Consulting

Oracle DBA Hosting

Oracle License Negotiation









Basic Direct Access Method (BDAM)

Oracle Tips by Burleson Consulting

As more data was being stored on disks, the direct access technique began to become more popular.  Unlike physical sequential, the BDAM (pronounced bee-damn) method uses a symbolic key, (which is usually a part of the record that is being stored) The BDAM hashing algorithm will compute the target location of the record on the disk and then tell the access method to get the record at that location.  Since we can go directly to the record, BDAM provides much faster access and retrieval of records.  A direct access file is sometimes called a “keyed” file because the key is used to generate the disk address (Figure 2-2).

Figure 2-2 Hashed file storage

Just as your home address uniquely identifies where you live, a disk address identifies where a record lives on the device.  A disk address includes the disk number, cylinder address, the track, and the block address.  To find a record in a direct access file, the program must have a key provided so that  it can determine the record address and then retrieve the desired record, regardless of all the other records in the file.  This was the first time that there had been a separation between the physical access methods and the logical access methods.  That is, the record could be retrieved simply by providing the key field, and the user need not be concerned with where the record was physically located on the disk.

Unfortunately, the range of addresses which were generated by the hashing algorithm required careful management. A hashing algorithm has to be repeatable, in that the same key always returns the same disk address.  BDAM has to be able to store a record with the hashing algorithm and be able to retrieve the record using the same algorithm.

Of course, duplicate keys must be avoided.  The duplication of disk addresses leads to “collisions” whereby the hashing algorithm cannot differentiate between two records.  Hence, it is critical that a unique key be created for each and every record in the file.  If a single field cannot uniquely identify the records, then a combination of fields or portions of fields can be used.  Ever see the code on a magazine subscription label?  These are called “concatenated” keys, and they contain pieces of fields in the base record, such as the first four letters of the subscriber’s last name, and a portion of the zip code.  Fortunately, there are many ways to generate unique keys. 

BDAM file structures also consume a large amount of disk storage. BDAM files do not make efficient use of disk storage since records are randomly distributed across the disk device.  Consequently, it is common to see hashed files with more unused spaces than occupied space see Figure 2-3.

Figure 2-3 The random distribution of BDAM records

In most cases, a BDAM file is considered "logically" full if more than 70 percent of the disk storage space contains data records. The upper and lower address limits for BDAM can be controlled, such that records can be stored on a single cylinder, a group of cylinders, or a group of disk devices.

Despite the problems of space usage and unique keys, hashing remains one of the fastest ways to store and retrieve information.  Almost all mainframe systems can take a symbolic key and convert it into a disk storage address in as little as 50 milliseconds.  Although hashing is a very old technique, it is still an extremely powerful method.  Many C++ programmers use hashing to store and retrieve records within their object-oriented applications and database objects require the definition of an object ID (OID), which is the unique address of the database object.

If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.

Expert Remote DBA

BC is America's oldest and largest Remote DBA Oracle support provider.  Get real Remote DBA experts, call
BC Remote DBA today.



Remote DBA Service

Oracle Tuning Book


Advance SQL Tuning Book 

BC Oracle support

Oracle books by Rampant

Oracle monitoring software







BC Remote Oracle Support

Remote DBA

Remote DBA Services

Copyright © 1996 -  2013 by Burleson. All rights reserved.

Oracle® is the registered trademark of Oracle Corporation.

Hit Counter