Physical Files and Logical Files

Learning Objective

By the end of this lesson, you will understand what a physical file is, what a logical file is, how they relate to each other, and how this connects to Db2 for i.

Simple Explanation

A physical file is where actual data is stored on IBM i. It holds records, and each record is made up of fields, similar in spirit to how a row is made up of columns in a table, though IBM i uses its own terminology for this.

A logical file is commonly used as an alternate view, or access path, over data that already lives in a physical file. Rather than storing a separate copy of the business data itself, a logical file normally presents the same underlying data in a different way, such as in a different order or through a different set of fields.

This connects directly to Lesson 3's mention of Db2 for i: physical files are the kind of underlying data storage that Db2 for i, the integrated database built into IBM i, works with.

Keyed access is a related idea worth introducing conceptually here. A key is one or more fields used to look up or order records in a meaningful way, similar to looking something up by a specific piece of information rather than reading through every record in whatever order it happens to be stored.

Why It Matters

RPG programs and other IBM i applications commonly read and write data through physical and logical files. Understanding the difference matters because it helps you recognize that you may be looking at different views of the same underlying data, rather than assuming every file you encounter holds its own independent copy of the data.

Practical Example

Imagine a physical file that stores customer order records. One logical file could present those same order records keyed by order date, useful for a shipping-related view. A second logical file could present the same order records keyed by customer number, useful for a customer-service view.

Both logical files are working with the same underlying order data stored in the physical file; they simply offer different ways of looking at and finding that data. This is a simplified, illustrative example rather than a real system, but it reflects a common pattern for organizing access to the same data in different ways.

Common Confusions

"Does a logical file duplicate the data from the physical file?" Normally, no. A logical file is generally used as a view or access path over the data in a physical file, rather than as a place where a separate duplicate copy of that business data is stored.

"Are records and fields the same as rows and columns in a spreadsheet?" Conceptually similar in some ways, but not identical. IBM i uses its own terminology (records and fields) for this idea, and the details of how they work go beyond what a spreadsheet analogy fully captures.

"Do I need to know DDS or SQL syntax to understand this lesson?" No. This lesson stays at a conceptual level. The specific syntax used to define physical and logical files is a more technical topic beyond the scope of this fundamentals lesson.

Quick Recap

• A physical file stores actual data on IBM i, organized as records made up of fields.
• A logical file is commonly used as an alternate view or access path over data that already lives in a physical file.
• A logical file normally does not store a separate duplicate copy of the underlying business data.
• Keyed access means looking up or ordering records by one or more specific fields, rather than reading through everything in storage order.
• Physical files are the kind of underlying data storage that Db2 for i, IBM i's integrated database, works with.

Try Asking the AI Tutor

Use the AI Tutor to ask follow-up questions about this lesson. For example, try asking:

• "What's the difference between a physical file and a logical file on IBM i?"