Reduce allocations in nested Arrow JSON rendering

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Reduce kernel-backend JSON rendering allocations for nested Boolean and signed-integer values.
 - **Behavior change:** Native Arrow TIMESTAMP_NTZ results now preserve their wall-clock fields and report `TIMESTAMP_NTZ` on both backends. Returned `time.Time` values use UTC instead of the session location; their represented instant and `.UTC()` value are unchanged (databricks/databricks-sql-go#506). On Thrift, this requires a server-supplied Arrow schema with native timestamps; without it, NTZ still reports `TIMESTAMP` and uses session-location handling. String timestamps (`UseArrowNativeTimestamp=false`, internal) and non-Arrow values also retain legacy parsing.
 
 ## v1.18.0 (2026-10-24)

internal/arrowscan/arrowscan.go

@@ -23,6 +23,7 @@ import (
 	"database/sql/driver"
 	"encoding/json"
 	"fmt"
+	"strconv"
 	"strings"
 	"time"
 
@@ -433,6 +434,21 @@ func writeJSON(b *strings.Builder, col arrow.Array, row int, loc *time.Location,
 		return nil
 	}
 	switch c := col.(type) {
+	case *array.Boolean:
+		if c.Value(row) {
+			b.WriteString("true")
+		} else {
+			b.WriteString("false")
+		}
+		return nil
+	case *array.Int8:
+		return writeIntJSON(b, int64(c.Value(row)))
+	case *array.Int16:
+		return writeIntJSON(b, int64(c.Value(row)))
+	case *array.Int32:
+		return writeIntJSON(b, int64(c.Value(row)))
+	case *array.Int64:
+		return writeIntJSON(b, c.Value(row))
 	case *array.List:
 		// Use the offset-aware ValueOffsets, NOT Offsets()[row]: arrow-go's
 		// Offsets() returns the full un-sliced buffer, so a List with a non-zero
@@ -479,6 +495,12 @@ func writeJSON(b *strings.Builder, col arrow.Array, row int, loc *time.Location,
 	}
 }
 
+func writeIntJSON(b *strings.Builder, v int64) error {
+	var scratch [20]byte // Sign and decimal digits of math.MinInt64 fit.
+	b.Write(strconv.AppendInt(scratch[:0], v, 10))
+	return nil
+}
+
 func writeListJSON(b *strings.Builder, values arrow.Array, start, end int, loc *time.Location, keys *StructKeyCache) error {
 	b.WriteByte('[')
 	for i := start; i < end; i++ {

internal/rows/arrowbased/arrowscan_parity_test.go

@@ -1,6 +1,7 @@
 package arrowbased
 
 import (
+	"strings"
 	"testing"
 	"time"
 
@@ -253,6 +254,30 @@ func TestArrowbasedKernelRenderParity(t *testing.T) {
 		name  string
 		build func() arrow.Array
 	}{
+		{"sliced_struct_scalar_leaves", func() arrow.Array {
+			dt := arrow.StructOf(
+				arrow.Field{Name: "flag", Type: arrow.FixedWidthTypes.Boolean, Nullable: true},
+				arrow.Field{Name: "tiny", Type: arrow.PrimitiveTypes.Int8, Nullable: true},
+				arrow.Field{Name: "small", Type: arrow.PrimitiveTypes.Int16, Nullable: true},
+				arrow.Field{Name: "int", Type: arrow.PrimitiveTypes.Int32, Nullable: true},
+				arrow.Field{Name: "big", Type: arrow.PrimitiveTypes.Int64, Nullable: true},
+			)
+			input := `[
+				{"flag":true,"tiny":1,"small":2,"int":3,"big":4},
+				{"flag":true,"tiny":-128,"small":-32768,"int":-2147483648,"big":-9223372036854775808},
+				{"flag":false,"tiny":127,"small":32767,"int":2147483647,"big":9223372036854775807},
+				{"flag":true,"tiny":-1,"small":-1,"int":-1,"big":-1},
+				{"flag":false,"tiny":0,"small":0,"int":0,"big":0},
+				{"flag":null,"tiny":null,"small":null,"int":null,"big":null},
+				null
+			]`
+			full, _, err := array.FromJSON(pool, dt, strings.NewReader(input), array.WithUseNumber())
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer full.Release()
+			return array.NewSlice(full, 1, int64(full.Len()))
+		}},
 		{"sliced_list", func() arrow.Array {
 			b := array.NewListBuilder(pool, arrow.PrimitiveTypes.Int64)
 			vb := b.ValueBuilder().(*array.Int64Builder)