[GitHub] [arrow-datafusion] WinkerDu commented on a diff in pull request #2183: MINOR: use arrow kernel `take` to avoid value copy in `string_concat`

[GitBox <gi...@apache.org>]

WinkerDu commented on code in PR #2183:
URL: https://github.com/apache/arrow-datafusion/pull/2183#discussion_r846830459


##########
datafusion/physical-expr/src/expressions/binary.rs:
##########
@@ -430,17 +431,17 @@ fn string_concat(left: ArrayRef, right: ArrayRef) -> Result<ArrayRef> {
         scalar_value => scalar_value.into_array(left.clone().len()),
     };
     let ignore_null_array = ignore_null.as_any().downcast_ref::<StringArray>().unwrap();
-    let result = (0..ignore_null_array.len())
+    let index_array = (0..ignore_null_array.len())
         .into_iter()
         .map(|index| {
             if left.is_null(index) || right.is_null(index) {
                 None
             } else {
-                Some(ignore_null_array.value(index))
+                Some(index as u32)
             }
         })
-        .collect::<StringArray>();
-
+        .collect::<UInt32Array>();
+    let result = take(ignore_null_array, &index_array, None)?;

Review Comment:
   I think we can optimize the whole string concat process to avoid this value copying, something like:
   
   - original process: build-in `concat` ignoring `NULL` -> generate validity array or array of indexes -> take valid value from `concat` output array according to bitmap or indexes
   - opmized process: concat two input string array well handled with `NULL`, no value copy any more.



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