wangrunji0408 commented on issue #11413:
URL: https://github.com/apache/datafusion/issues/11413#issuecomment-2242629113
> > An example would be
>
> FWIW this implementation of concat would likely perform pretty poorly
compared to a hand written one as 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)
> }
> ```
In this case, a writer-style return value is supported to avoid the overhead.
```rust
#[function("concat(string, string) -> string")]
#[function("concat(largestring, largestring) -> largestring")] // will be
supported soon
fn concat(lhs: &str, rhs: &str, output: &mut impl std::fmt::Write) {
write!(output, "{}{}", lhs, rhs).unwrap();
}
```
The same technique can be applied to `binary` and `largebinary`. However, it
is not yet implemented due to the lack of `impl std::io::Write for
(Large)BinaryBuilder` in arrow crate.
```rust
#[function("concat(binary, binary) -> binary")]
#[function("concat(largebinary, largebinary) -> largebinary")]
fn concat(lhs: &[u8], rhs: &[u8], output: &mut impl std::io::Write) {
output.write_all(lhs).unwrap();
output.write_all(rhs).unwrap();
}
```
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