Write small decimals as INTs (#1983)

# Rationale for this change

Resolves #1979

# Are these changes tested?

# Are there any user-facing changes?

<!-- In the case of user-facing changes, please add the changelog label.
-->

Author: Fokko

pyiceberg/io/pyarrow.py

@@ -636,6 +636,12 @@ def visit_fixed(self, fixed_type: FixedType) -> pa.DataType:
         return pa.binary(len(fixed_type))
 
     def visit_decimal(self, decimal_type: DecimalType) -> pa.DataType:
+        # It looks like decimal{32,64} is not fully implemented:
+        # https://github.com/apache/arrow/issues/25483
+        # https://github.com/apache/arrow/issues/43956
+        # However, if we keep it as 128 in memory, and based on the
+        # precision/scale Arrow will map it to INT{32,64}
+        # https://github.com/apache/arrow/blob/598938711a8376cbfdceaf5c77ab0fd5057e6c02/cpp/src/parquet/arrow/schema.cc#L380-L392
         return pa.decimal128(decimal_type.precision, decimal_type.scale)
 
     def visit_boolean(self, _: BooleanType) -> pa.DataType:
@@ -2442,7 +2448,9 @@ def write_parquet(task: WriteTask) -> DataFile:
         )
         fo = io.new_output(file_path)
         with fo.create(overwrite=True) as fos:
-            with pq.ParquetWriter(fos, schema=arrow_table.schema, **parquet_writer_kwargs) as writer:
+            with pq.ParquetWriter(
+                fos, schema=arrow_table.schema, store_decimal_as_integer=True, **parquet_writer_kwargs
+            ) as writer:
                 writer.write(arrow_table, row_group_size=row_group_size)
         statistics = data_file_statistics_from_parquet_metadata(
             parquet_metadata=writer.writer.metadata,

tests/integration/test_writes/test_writes.py

@@ -20,6 +20,7 @@
 import random
 import time
 from datetime import date, datetime, timedelta
+from decimal import Decimal
 from pathlib import Path
 from typing import Any, Dict
 from urllib.parse import urlparse
@@ -50,6 +51,7 @@
 from pyiceberg.transforms import DayTransform, HourTransform, IdentityTransform
 from pyiceberg.types import (
     DateType,
+    DecimalType,
     DoubleType,
     IntegerType,
     ListType,
@@ -1810,3 +1812,32 @@ def test_evolve_and_write(
         )
 
     assert session_catalog.load_table(identifier).scan().to_arrow().column(0).combine_chunks() == numbers
+
+
+@pytest.mark.integration
+def test_read_write_decimals(session_catalog: Catalog) -> None:
+    """Roundtrip decimal types to make sure that we correctly write them as ints"""
+    identifier = "default.test_read_write_decimals"
+
+    arrow_table = pa.Table.from_pydict(
+        {
+            "decimal8": pa.array([Decimal("123.45"), Decimal("678.91")], pa.decimal128(8, 2)),
+            "decimal16": pa.array([Decimal("12345679.123456"), Decimal("67891234.678912")], pa.decimal128(16, 6)),
+            "decimal19": pa.array([Decimal("1234567890123.123456"), Decimal("9876543210703.654321")], pa.decimal128(19, 6)),
+        },
+    )
+
+    tbl = _create_table(
+        session_catalog,
+        identifier,
+        properties={"format-version": 2},
+        schema=Schema(
+            NestedField(1, "decimal8", DecimalType(8, 2)),
+            NestedField(2, "decimal16", DecimalType(16, 6)),
+            NestedField(3, "decimal19", DecimalType(19, 6)),
+        ),
+    )
+
+    tbl.append(arrow_table)
+
+    assert tbl.scan().to_arrow() == arrow_table