findepi commented on issue #11413:
URL: https://github.com/apache/datafusion/issues/11413#issuecomment-2347153946
> it will both create and allocate a new temporary `String` for each row
(and then presumably copy that value into a final StringArray/LargeStringArray).
>
> ```rust
> // declare concat
> #[function("concat(string, string) -> string")]
> #[function("concat(largestring, largestring) -> largestring")]
> fn concat(lhs: &str, rhs: &str) -> String {
> format!("{}{}", lhs, rhs)
> }
> ```
I understand we're not concerned whether `format!` is the best way to
concatenate two strings, but more about the fundamental fact that `concat`
function allocates and returns a new `String` object every time.
I don't know how does `#[function]` macro work, but I assume it generates a
ColumnarValue processor that handles ScalarValue directly and an Array value
with a for loop. For the for loop code branch, is the compiler capable of
optimizing away the `concat`'s allocation?
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