Intel 170 Servers, AS/400 RISC Server, 7xx Servers manual Order1 Orderj Ordert OrderN, Brief Example

Order(1)	Order(j)	Order(t)	Order(N)
ILE and OS/400	Just In Time compiled	Java threads	Data Base Records and
Programs	programs (Java *JIT)		IFS file records
Subsystem Descriptions	Total Job Storage	File Buffers of all kinds	Java (and C/C++)
			objects
Direct Execution Java	Static storage from	SQL Result Set	Operating System
Programs	RPG and COBOL.	(nonrecord)	copies (e.g. Data Base)
	Static final in Java.		copies of application
			records
System values	Java Virtual Machine	Program stack storage	SQL records in a result
	and most WebSphere		set
	storage

A Brief Example

To show these concepts, consider a simple example.

Part of a financial system has three logical elements to deal with:

1.An order record (order summary including customer information, sales tax, etc.)

2.An order detail record (individual purchased items, quantities, prices).

3.A table containing international currency rates of exchange between two arbitrary countries.

Question: What is more important? Reducing the cost of the detail record by a couple of bytes, or reducing the currency table from a cost of N squared (where “N” is the number of countries) to 2 times N.

There are two obvious implementations of the currency table:

1.Implement the table as a two dimensional array such that CurrencyExchangei,j will give the exchange between countryi and countryj for all countries.

2.Implement the table as a single dimension array with the ith element being the exchange rate between countryi and the US dollar. One can convert to any country simply by converting twice; once to dollars and once to the other currency.

Clearly, the second is more storage efficient.

Now consider the first problem. The detail record looks like this:

Quantity as a four byte number (9B or 10B in RPG terms). Name of the item (up to 60 characters)

Price of the item (as a zoned decimal field, 15 total digits with two decimal points).

A simple scrub would give:

Quantity as a two byte number (4B in RPG terms). Name of the item (probably still 60 characters)

Price of the item (as a packed decimal field, probably 10 total digits with two decimal points).

IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
© Copyright IBM Corp. 2008	Chapter 20 - General Tips and Techniques	318