felipecrv commented on issue #41094:
URL: https://github.com/apache/arrow/issues/41094#issuecomment-2087716483
> Because if-else related expressions have a special execution order, should
we extend ExecuteScalarIfElseExpression to handle such expressions separately?
The solution should not be specific to if-then-else. Other constructs pose
similar challenges (e.g. `CASE WHEN`):
Velox (a vectorized query engine) calls these constructs "special forms".
LISP uses the same terminology. Although `if` and `cond` look like function
calls in Lisp [1][2], they are special in that not all arguments are eagerly
evaluated:
```lisp
(if (> den 0) (/ num den) 0)
```
Other special forms are `and`, `or`, and `cond`:
```
(and expr ...)
(or expr ...)
(cond [test-expr body ...+]
...)
```
`and` is special because after the first `false` you get, you don't have to
evaluate more input expressions. `or` is special because after the first `true`
you get, you... `cond` is like `CASE .. WHEN` in SQL — a chain of
if-then-else's.
This can't be solved when `if_then_else` and `case_when` **are implemented
as functions** in Arrow. We need to create special expression fragments that
can receive unevaluated expressions as input parameters.
They would look like `compute::Expression::Call` that can receive
`compute::Expression` instances as parameters.
https://github.com/apache/arrow/blob/0d7fac0d49eae7f139735c3e7c9256fc304a698a/cpp/src/arrow/compute/expression.h#L136
```diff
- using Impl = std::variant<Datum, Parameter, Call>;
+ using Impl = std::variant<Datum, Parameter, Call, Special>;
```
### Vectorized Evaluation of Special Forms
Here I will try to expand on what @bkietz meant by "support for selection
vectors".
[1] https://courses.cs.northwestern.edu/325/readings/special-forms.html
[2] https://docs.racket-lang.org/guide/conditionals.html
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