SQE vs CQE at a Beginner-Friendly Level

Learning Objective

By the end of this lesson, you will be able to explain, at a beginner-friendly level, what SQE and CQE are and why Db2 for i has two query engines.

Simple Explanation

The previous lesson introduced the optimizer as the part of Db2 for i that decides how a query actually retrieves its rows. Db2 for i actually has two different query engines capable of doing this work:

• CQE (Classic Query Engine): the older of the two, originally built to handle both DDS-described files and SQL tables, including older or more unusual query patterns.
• SQE (SQL Query Engine): a newer engine, built specifically around SQL tables and modern SQL features, generally able to make better optimization decisions for typical SQL queries.

For most modern SQL queries against SQL-created or well-described tables, Db2 for i uses SQE automatically; certain older patterns or DDS-specific cases may still be handled by CQE.

Why It Matters

Knowing that two engines exist explains why some query behavior or performance advice a developer reads may not apply universally: it may be specific to one engine or the other. At a beginner-friendly level, the important takeaway is simply that this distinction exists and why it exists, not memorizing every rule for which engine handles which case.

Practical Example

Recall the CUSTMAST table used throughout this batch, created with SQL DDL. A typical modern SELECT against it, like the queries used in earlier lessons in this batch, is the kind of query SQE is built to optimize well. An older query pattern against a DDS-described file with unusual, legacy characteristics is more the kind of case CQE was originally built to continue supporting.

This is a simplified, illustrative example rather than a specific real performance comparison, but it reflects why Db2 for i maintains two engines rather than one.

Common Confusions

"Do I need to choose between SQE and CQE myself when writing a query?" No, not at this introductory level. Db2 for i decides which engine handles a given query automatically. Understanding that this decision happens, and roughly why, is the goal of this lesson, not manually choosing an engine.

"Is CQE outdated and about to be removed?" This lesson does not make claims about IBM's long-term plans for either engine; what matters here is understanding that both currently exist and serve different historical purposes, which is enough context for a beginner-friendly introduction.

"Does knowing about SQE and CQE help me write better SQL day to day?" Mostly indirectly: it explains why some performance advice you might encounter is engine-specific rather than universal. Writing well-formed SQL against properly indexed SQL tables, already covered in this batch, matters more day to day than tracking which engine handles a specific query.

Quick Recap

• Db2 for i has two query engines: CQE, the older engine, and SQE, the newer engine built around modern SQL and SQL tables.
• Db2 for i decides automatically which engine handles a given query; this is not something a developer chooses manually.
• Most modern SQL queries against SQL tables are handled by SQE.
• The main value of knowing this distinction is understanding that some performance advice is engine-specific, not universal.

Try Asking the AI Tutor

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

• "Why might a shop still have query patterns that end up handled by CQE instead of SQE?"
• "Does the SQE versus CQE distinction matter for the kind of everyday SQLRPGLE queries covered earlier in this course?"