perf: Optimize concat()/concat_ws() UDFs (#20317)

## Which issue does this PR close?

- Closes #20316.

## Rationale for this change

Faster is better.

## What changes are included in this PR?

This commit implements three optimizations:

* In `StringViewArrayBuilder`, we recreated `block` after every call to
`append_offset`. It is cheaper to instead clear and re-use `block`.

* In `StringViewArrayBuilder::write()`, we re-validated that a string
array consists of valid UTF8 characters. This was unnecessary work and
can be skipped.

* In the concat() UDF implementation, we miscalculated the initial size
of the StringViewArrayBuilder buffer. This didn't lead to incorrect
behavior but it resulted in unnecessarily needing to reallocate the
buffer.

## Are these changes tested?

Yes; no additional test cases warranted.

## Are there any user-facing changes?

No.

Author: neilconway

datafusion/functions/src/string/concat.rs

@@ -204,7 +204,9 @@ impl ScalarUDFImpl for ConcatFunc {
                         DataType::Utf8View => {
                             let string_array = as_string_view_array(array)?;
 
-                            data_size += string_array.len();
+                            // This is an estimate; in particular, it will
+                            // undercount arrays of short strings (<= 12 bytes).
+                            data_size += string_array.total_buffer_bytes_used();
                             let column = if array.is_nullable() {
                                 ColumnarValueRef::NullableStringViewArray(string_array)
                             } else {

datafusion/functions/src/string/concat_ws.rs

@@ -247,6 +247,8 @@ impl ScalarUDFImpl for ConcatWsFunc {
                         DataType::Utf8View => {
                             let string_array = as_string_view_array(array)?;
 
+                            // This is an estimate; in particular, it will
+                            // undercount arrays of short strings (<= 12 bytes).
                             data_size += string_array.total_buffer_bytes_used();
                             let column = if array.is_nullable() {
                                 ColumnarValueRef::NullableStringViewArray(string_array)

datafusion/functions/src/strings.rs

@@ -152,43 +152,34 @@ impl StringViewArrayBuilder {
             }
             ColumnarValueRef::NullableArray(array) => {
                 if !CHECK_VALID || array.is_valid(i) {
-                    self.block.push_str(
-                        std::str::from_utf8(array.value(i).as_bytes()).unwrap(),
-                    );
+                    self.block.push_str(array.value(i));
                 }
             }
             ColumnarValueRef::NullableLargeStringArray(array) => {
                 if !CHECK_VALID || array.is_valid(i) {
-                    self.block.push_str(
-                        std::str::from_utf8(array.value(i).as_bytes()).unwrap(),
-                    );
+                    self.block.push_str(array.value(i));
                 }
             }
             ColumnarValueRef::NullableStringViewArray(array) => {
                 if !CHECK_VALID || array.is_valid(i) {
-                    self.block.push_str(
-                        std::str::from_utf8(array.value(i).as_bytes()).unwrap(),
-                    );
+                    self.block.push_str(array.value(i));
                 }
             }
             ColumnarValueRef::NonNullableArray(array) => {
-                self.block
-                    .push_str(std::str::from_utf8(array.value(i).as_bytes()).unwrap());
+                self.block.push_str(array.value(i));
             }
             ColumnarValueRef::NonNullableLargeStringArray(array) => {
-                self.block
-                    .push_str(std::str::from_utf8(array.value(i).as_bytes()).unwrap());
+                self.block.push_str(array.value(i));
             }
             ColumnarValueRef::NonNullableStringViewArray(array) => {
-                self.block
-                    .push_str(std::str::from_utf8(array.value(i).as_bytes()).unwrap());
+                self.block.push_str(array.value(i));
             }
         }
     }
 
     pub fn append_offset(&mut self) {
         self.builder.append_value(&self.block);
-        self.block = String::new();
+        self.block.clear();
     }
 
     pub fn finish(mut self) -> StringViewArray {