pepijnve commented on code in PR #122:
URL: https://github.com/apache/datafusion-site/pull/122#discussion_r2640113027
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content/blog/2025-11-11-datafusion_case.md:
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+---
+layout: post
+title: Optimizing CASE Expression Evaluation
+date: 2025-11-11
+author: Pepijn Van Eeckhoudt
+categories: [features]
+---
+<!--
+{% comment %}
+Licensed to the Apache Software Foundation (ASF) under one or more
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+The ASF licenses this file to you under the Apache License, Version 2.0
+(the "License"); you may not use this file except in compliance with
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+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
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+{% endcomment %}
+-->
+
+[TOC]
+
+<style>
+figure {
+ margin: 20px 0;
+}
+
+figure img {
+ display: block;
+ max-width: 80%;
+ margin: auto;
+}
+
+figcaption {
+ font-style: italic;
+ color: #555;
+ font-size: 0.9em;
+ max-width: 80%;
+ margin: auto;
+ text-align: center;
+}
+</style>
+
+# Optimizing CASE Expression Evaluation in DataFusion
+
+SQL's `CASE` expression is one of the few constructs the language provides to
perform conditional logic.
+Its deceptively simple syntax hides significant implementation complexity.
+Over the past few weeks, we've landed a series of improvements to DataFusion's
`CASE` expression evaluator that reduce both CPU time and memory allocations.
+This post walks through the original implementation, its performance
bottlenecks, and how we addressed them step by step.
+Finally we'll also take a look at some future improvements to `CASE` that are
in the works.
+
+## Background: CASE Expression Evaluation
+
+SQL supports two forms of CASE expressions:
+
+1. **Simple**: `CASE expr WHEN value1 THEN result1 WHEN value2 THEN result2
... END`
+2. **Searched**: `CASE WHEN condition1 THEN result1 WHEN condition2 THEN
result2 ... END`
+
+The simple form evaluates an expression once for each input row and then tests
that value against the constants (or expressions) in each `WHEN` clause using
equality comparisons.
+Think of it as a limited Rust `match` expression.
+
+Here's a simple example:
+
+```sql
+CASE status
+ WHEN 'pending' THEN 1
+ WHEN 'active' THEN 2
+ WHEN 'complete' THEN 3
+ ELSE 0
+END
+```
+
+In this `CASE` expression, `status` is evaluated once per row, and then its
value is tested for equality with the values `'pending'`, `'active'`, and
`'complete'` in that order.
+The `THEN` expression value for the first matching `WHEN` expression is
returned per row.
+
+The searched `CASE` form is a more flexible variant.
+It evaluates completely independent boolean expressions for each branch.
+
+This allows you to test different columns with different operators per branch
as can be seen in the following example:
+
+```sql
+CASE
+ WHEN age > 65 THEN 'senior'
+ WHEN childCount != 0 THEN 'parent'
+ WHEN age < 21 THEN 'minor'
+ ELSE 'adult'
+END
+```
+
+In both forms, branches are evaluated sequentially with short-circuit
semantics: for each row, once a `WHEN` condition matches, the corresponding
`THEN` expression is evaluated. Any further branches are not evaluated for that
row.
+This lazy evaluation model is critical for correctness.
+It let's you safely write `CASE` expressions like
+
+```sql
+CASE
+ WHEN denominator == 0 THEN NULL
+ ELSE nominator / denominator
+END
+```
+
+that are guaranteed to not trigger divide-by-zero errors.
+
+## `CASE` Evaluation in DataFusion 50.0.0
+
+For the rest of this post we'll be looking at 'searched case' evaluation.
+'Simple case' uses a distinct, but very similar implementation.
+The same set of improvements has been applied to both.
Review Comment:
I'll see if I can polish the quick sketches I made when I was working on
this. Copying them here as reference.
Is something along these lines what you have in mind?
<img width="2510" height="2256" alt="image"
src="https://github.com/user-attachments/assets/312d1264-0abc-426f-ae8d-819835d2632f";
/>
<img width="2092" height="2374" alt="image"
src="https://github.com/user-attachments/assets/aa1e5f5f-0877-4617-8fa4-81d9bbdc5f17";
/>
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