EeshanBembi opened a new pull request, #17450:
URL: https://github.com/apache/datafusion/pull/17450
## Summary
Adds an optimization rule to simplify CASE expressions where the first
condition is always true (literal `true`), reducing them to just the THEN
expression.
This fixes the issue where expressions like:
```sql
CASE WHEN true THEN 1 ELSE x END
```
were not being simplified to just `1`.
## Changes
- Added new simplification rule in `expr_simplifier.rs` that detects `CASE
WHEN true THEN expr` patterns
- Added comprehensive tests covering various scenarios including:
- Simple literal values
- Column references
- Multiple WHEN clauses (only first is simplified)
- Cases with and without ELSE clauses
## Test Plan
- [x] Added unit tests for the new simplification rule
- [x] Verified existing CASE tests still pass
- [x] Tested with the exact example from the issue
- [x] Confirmed logical and physical plans now show simplified expressions
## Before/After
**Before:**
```
logical_plan | Projection: CASE WHEN Boolean(true) THEN Int64(1) ELSE
CAST(foo.x AS Int64) END
physical_plan | ProjectionExec: expr=[CASE WHEN true THEN 1 ELSE CAST(x@0 AS
Int64) END as ...]
```
**After:**
```
logical_plan | Projection: Int64(1) AS CASE WHEN Boolean(true) THEN
Int64(1) ELSE foo.x END
physical_plan | ProjectionExec: expr=[1 as CASE WHEN Boolean(true) THEN
Int64(1) ELSE foo.x END]
```
Fixes #17448
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