[SS-166] Widen dimension : u8 to i32 when writing Array values to Iceberg.

src/arrow-util/src/builder.rs

@@ -1034,13 +1034,21 @@ impl ArrowColumn {
                     )
                 }
                 let dims_builder: &mut ArrowColumn = struct_builder.field_builder(1).unwrap();
-                if let ColBuilder::UInt8Builder(dims_builder) = &mut dims_builder.inner {
-                    dims_builder.append_value(arr.dims().ndims());
-                } else {
-                    anyhow::bail!(
-                        "Expected UInt8Builder for StructBuilder with Array datum: {:?}",
+                match &mut dims_builder.inner {
+                    ColBuilder::UInt8Builder(dims_builder) => {
+                        dims_builder.append_value(arr.dims().ndims());
+                    }
+                    // Iceberg has no narrow integer types, so the synthetic
+                    // `dimensions` field comes back from the Iceberg schema
+                    // widened to Int32. Promote `ndims` (a u8) the same way
+                    // smallint columns widen into an Int32Builder.
+                    ColBuilder::Int32Builder(dims_builder) => {
+                        dims_builder.append_value(i32::from(arr.dims().ndims()));
+                    }
+                    _ => anyhow::bail!(
+                        "Expected UInt8Builder or Int32Builder for StructBuilder with Array datum: {:?}",
                         struct_builder
-                    )
+                    ),
                 }
                 struct_builder.append(true)
             }

test/iceberg/catalog.td

@@ -335,3 +335,37 @@ SELECT id, cardinality(props), list_sort(map_keys(props))::VARCHAR, list_sort(ma
 1 2 [a, b] [bar, foo]
 2 0 [] []
 3 1 [key] [value]
+
+# Test array columns
+# Materialize arrays are an arrow struct of {items: list, dimensions: uint8}.
+# Iceberg has no uint8, so the dimensions field widens to Iceberg int (Int32),
+# the same way smallint does above. The array Datum's ndims (a u8) must promote
+# into that Int32Builder, or the sink stalls converting the row to a recordbatch
+# with "Expected UInt8Builder for StructBuilder with Array datum".
+> CREATE TABLE arrays(id int, vals int[]);
+
+> INSERT INTO arrays VALUES
+    (1, '{1,2,3}'),
+    (2, '{}'),
+    (3, '{42}');
+
+> CREATE SINK array_demo
+    FROM arrays
+    INTO ICEBERG CATALOG CONNECTION polaris (
+        NAMESPACE 'default_namespace',
+        TABLE 'array_table'
+    )
+    USING AWS CONNECTION aws_conn
+    KEY (id) NOT ENFORCED
+    MODE UPSERT
+    WITH (COMMIT INTERVAL '1s');
+
+$ sleep-is-probably-flaky-i-have-justified-my-need-with-a-comment duration=10s
+
+# The array lands as a struct {items: list, dimensions: int}. Verify the
+# elements and dimension count round-trip.
+$ duckdb-query name=iceberg
+SELECT id, vals.items::VARCHAR, vals.dimensions FROM iceberg_scan('s3://test-bucket/default_namespace/array_table') ORDER BY id
+1 [1, 2, 3] 1
+2 [] 0
+3 [42] 1