kosiew opened a new pull request, #18191:
URL: https://github.com/apache/datafusion/pull/18191
## Which issue does this PR close?
* Closes #17983
## Rationale for this change
The current implementation of the `nvl2` function in DataFusion eagerly
evaluates all its arguments, which can lead to unnecessary computation and
incorrect behavior when handling expressions that should only be conditionally
evaluated. This PR introduces **lazy evaluation** for `nvl2`, aligning its
behavior with other conditional expressions like `coalesce` and improving both
performance and correctness.
This change also introduces a **simplification rule** that rewrites `nvl2`
expressions into equivalent `CASE` statements, allowing for better optimization
during query planning and execution.
## What changes are included in this PR?
* Refactored `nvl2` implementation in
`datafusion/functions/src/core/nvl2.rs`:
* Added support for **short-circuit (lazy) evaluation** using
`short_circuits()`.
* Implemented **simplify()** method to rewrite expressions into `CASE`
form.
* Introduced **return_field_from_args()** for correct nullability and type
inference.
* Replaced the previous eager `nvl2_func()` logic with an optimized, more
declarative approach.
* Added comprehensive **unit tests**:
* `test_nvl2_short_circuit` in `dataframe_functions.rs` verifies correct
short-circuit behavior.
* `test_create_physical_expr_nvl2` in `expr_api/mod.rs` validates physical
expression creation and output correctness.
## Are these changes tested?
✅ Yes, multiple new tests are included:
* **`test_nvl2_short_circuit`** ensures `nvl2` does not evaluate unnecessary
branches.
* **`test_create_physical_expr_nvl2`** checks the correctness of evaluation
and type coercion behavior.
All existing and new tests pass successfully.
## Are there any user-facing changes?
Yes, but they are **non-breaking** and **performance-enhancing**:
* `nvl2` now evaluates lazily, meaning only the required branch is computed
based on the nullity of the test expression.
* Expression simplification will yield more optimized query plans.
There are **no API-breaking changes**. However, users may observe improved
performance and reduced computation for expressions involving `nvl2`.
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