From 518c00b33a24d093d0c7a3226b9db407c051a4f1 Mon Sep 17 00:00:00 2001
From: Mikhail Zabaluev <mikhail.zabaluev@gmail.com>
Date: Mon, 3 Aug 2026 20:52:10 +0300
Subject: [PATCH 1/2] feat: re-expose string builder API for concat

The builders hidden by https://github.com/apache/datafusion/pull/21695
are useful for third party UDFs that want to implement their own concat
functions, e.g. one that faithfully replicates the behavior of
Spark's concat.
---
 datafusion/functions/src/strings.rs | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/datafusion/functions/src/strings.rs b/datafusion/functions/src/strings.rs
index c788c6fb1f33f..28e955341bb36 100644
--- a/datafusion/functions/src/strings.rs
+++ b/datafusion/functions/src/strings.rs
@@ -39,7 +39,7 @@ use datafusion_common::cast::{
 use datafusion_expr_common::columnar_value::ColumnarValue;
 
 /// Trait abstracting concatenating string and binary collections.
-pub(crate) trait ConcatBuilder {
+pub trait ConcatBuilder {
     fn write<const CHECK_VALID: bool>(
         &mut self,
         column: &ColumnarValueRef,
@@ -61,13 +61,13 @@ pub(crate) trait ConcatBuilder {
 ///
 /// For the common "produce one `&str` per row" pattern, prefer
 /// `GenericStringArrayBuilder` instead.
-pub(crate) struct ConcatGenericStringBuilder<O: OffsetSizeTrait + ArrowNativeType> {
+pub struct ConcatGenericStringBuilder<O: OffsetSizeTrait + ArrowNativeType> {
     offsets_buffer: MutableBuffer,
     value_buffer: MutableBuffer,
     _phantom: PhantomData<O>,
 }
-pub(crate) type ConcatStringBuilder = ConcatGenericStringBuilder<i32>;
-pub(crate) type ConcatLargeStringBuilder = ConcatGenericStringBuilder<i64>;
+pub type ConcatStringBuilder = ConcatGenericStringBuilder<i32>;
+pub type ConcatLargeStringBuilder = ConcatGenericStringBuilder<i64>;
 
 impl<O: OffsetSizeTrait + ArrowNativeType> ConcatGenericStringBuilder<O> {
     pub fn with_capacity(item_capacity: usize, data_capacity: usize) -> Self {
@@ -187,7 +187,7 @@ impl<O: OffsetSizeTrait + ArrowNativeType> ConcatBuilder
 ///
 /// For the common "produce one `&str` per row" pattern, prefer
 /// [`StringViewArrayBuilder`] instead.
-pub(crate) struct ConcatStringViewBuilder {
+pub struct ConcatStringViewBuilder {
     views: Vec<u128>,
     data: Vec<u8>,
     block: Vec<u8>,
@@ -1217,7 +1217,7 @@ pub(crate) fn append_view(
 }
 
 #[derive(Debug)]
-pub(crate) enum ColumnarValueRef<'a> {
+pub enum ColumnarValueRef<'a> {
     Scalar(&'a [u8]),
     NullableArray(&'a StringArray),
     NonNullableArray(&'a StringArray),

From b7a8ce4735fd90ce79c0870e10f9543d73eb6144 Mon Sep 17 00:00:00 2001
From: Mikhail Zabaluev <mikhail.zabaluev@gmail.com>
Date: Tue, 4 Aug 2026 00:24:38 +0300
Subject: [PATCH 2/2] chore: fix doc comment

Remove a reference to a private type from the doc comment on
ConcatGenericStringBuilder, which is now public. Turn the paragraph
into a plain comment with a note to add it later if
StringViewArrayBuilder is made public.
---
 datafusion/functions/src/strings.rs | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/datafusion/functions/src/strings.rs b/datafusion/functions/src/strings.rs
index 28e955341bb36..58223e33d3815 100644
--- a/datafusion/functions/src/strings.rs
+++ b/datafusion/functions/src/strings.rs
@@ -184,9 +184,11 @@ impl<O: OffsetSizeTrait + ArrowNativeType> ConcatBuilder
 /// fragment) followed by a single `append_offset` to commit the row
 /// as a single string view. The output null buffer is supplied by the caller
 /// at `finish` time, avoiding per-row NULL handling work.
-///
-/// For the common "produce one `&str` per row" pattern, prefer
-/// [`StringViewArrayBuilder`] instead.
+//
+// Add below to the doc comment if StringViewArrayBuilder is made public:
+//
+// For the common "produce one `&str` per row" pattern, prefer
+// [`StringViewArrayBuilder`] instead.
 pub struct ConcatStringViewBuilder {
     views: Vec<u128>,
     data: Vec<u8>,