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Comparing a AS/400 and a mainframe is like comparing an apple and an orange. I will tell you what an AS/400 can do, and speak what I can for the mainframe. The AS/400 is one of the best database machines out there. It can now handle terabytes of data and retrieve records extremely fast. The AS/400 manages all "objects" on the system in a journal for fast retrieval and some really smart geeks at IBM developed a formula to find records in three tries or less. What this means is that DB2 will go directly to a "keyed" file and look for the record using a formula, if not found it will try again. Studies have found that no more then three tries by the OS where needed to find a
record. I have dealt with files that contain over 100 million records and can SQL, Query, or program read a record in milliseconds. This puppy is fast and when retrieved, all the data fields with in the record are validated. You can reduce the performance of DB2 by creating all types of logical views of the data, but it only hurts the write operation not the retrieval.
My limited experiences with mainframes are with flat record files that are not relational in nature. Although the retrieval is fast, the manipulation of the data can be complex, which reduces the overall speed of the data retrieval. Please remember that I am an AS/400 bigot, so I mean no harm to the "mainframers" and I cannot toot their horn well at all. I hope this helps some.
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