Improve implementation and do the right thing for null

datafusion/physical-expr/src/expressions/binary/kernels.rs

@@ -144,12 +144,22 @@ pub fn concat_elements_utf8view(
         .map(|(b1, b2)| b1.len() + b2.len())
         .sum();
     let mut result = StringViewBuilder::with_capacity(capacity);
+
+    // Avoid reallocations by writing to a reused buffer (note we
+    // could be even more efficient r by creating the view directly
+    // here and avoid the buffer but that would be more complex)
+    let mut buffer = String::new();
+
     for (left, right) in left.iter().zip(right.iter()) {
-        match (left, right) {
-            (None, None) => result.append_null(),
-            _ => {
-                result.append_value(left.unwrap_or("").to_string() + right.unwrap_or(""))
-            }
+        if let (Some(left), Some(right)) = (left, right) {
+            use std::fmt::Write;
+            buffer.clear();
+            write!(&mut buffer, "{left}{right}")
+                .expect("writing into string buffer failed");
+            result.append_value(&buffer);
+        } else {
+            // at least one of the values is null, so the output is also null
+            result.append_null()
         }
     }
     Ok(result.finish())

datafusion/sqllogictest/test_files/string_view.slt

@@ -1146,6 +1146,7 @@ FROM test;
 NULL
 
 # || mixed types
+# expect all results to be the same for each row as they all have the same values
 query TTTTTTTT
 SELECT
   column1_utf8view || column2_utf8view,
@@ -1162,9 +1163,10 @@ FROM test;
 AndrewX AndrewX AndrewX AndrewX XAndrew XAndrew XAndrew XAndrew
 XiangpengXiangpeng XiangpengXiangpeng XiangpengXiangpeng XiangpengXiangpeng XiangpengXiangpeng XiangpengXiangpeng XiangpengXiangpeng XiangpengXiangpeng
 RaphaelR RaphaelR RaphaelR RaphaelR RRaphael RRaphael RRaphael RRaphael
-R R R R R R R R
+NULL NULL NULL NULL NULL NULL NULL NULL
 
 # || constants
+# expect all results to be the same for each row as they all have the same values
 query TTTTTTTT
 SELECT
   column1_utf8view || 'foo',
@@ -1181,12 +1183,10 @@ FROM test;
 Andrewfoo Andrewfoo Andrewfoo Andrewfoo fooAndrew fooAndrew fooAndrew fooAndrew
 Xiangpengfoo Xiangpengfoo Xiangpengfoo Xiangpengfoo fooXiangpeng fooXiangpeng fooXiangpeng fooXiangpeng
 Raphaelfoo Raphaelfoo Raphaelfoo Raphaelfoo fooRaphael fooRaphael fooRaphael fooRaphael
-foo NULL NULL NULL foo NULL NULL NULL
-
-
-
+NULL NULL NULL NULL NULL NULL NULL NULL
 
 # || same type (column1 has null, so also tests NULL || NULL)
+# expect all results to be the same for each row as they all have the same values
 query TTT
 SELECT
   column1_utf8view || column1_utf8view,
@@ -1201,8 +1201,6 @@ XiangpengXiangpeng XiangpengXiangpeng XiangpengXiangpeng
 RaphaelRaphael RaphaelRaphael RaphaelRaphael
 NULL NULL NULL
 
-
-
 statement ok
 drop table test;