PARQUET-2264: Allow scale == precision for DecimalType

[devinrsmith]

The majority of implementations allow for scale == precision.

See apache/arrow-rs#1607 for further motivation.

[wgtmac]

Thanks for the PR! Could you also add the comment to the thrift definition: https://github.com/apache/parquet-format/blob/master/src/main/thrift/parquet.thrift#L253 ?

[devinrsmith]

Will do.

[mapleFU]

diff --git a/cpp/examples/parquet/parquet_arrow/reader_writer.cc b/cpp/examples/parquet/parquet_arrow/reader_writer.cc
index f5d96ec16..a4c4e5f77 100644
--- a/cpp/examples/parquet/parquet_arrow/reader_writer.cc
+++ b/cpp/examples/parquet/parquet_arrow/reader_writer.cc
@@ -27,10 +27,12 @@
 // To have some input data, we first create an Arrow Table that holds
 // some data.
 std::shared_ptr<arrow::Table> generate_table() {
-  arrow::Int64Builder i64builder;
-  PARQUET_THROW_NOT_OK(i64builder.AppendValues({1, 2, 3, 4, 5}));
-  std::shared_ptr<arrow::Array> i64array;
-  PARQUET_THROW_NOT_OK(i64builder.Finish(&i64array));
+  arrow::Decimal128Builder decimalBuilder(arrow::decimal(15, 15));
+  for (int i = 0; i < 5; ++i) {
+    PARQUET_THROW_NOT_OK(decimalBuilder.Append(arrow::Decimal128()));
+  }
+  std::shared_ptr<arrow::Array> decimalArray;
+  PARQUET_THROW_NOT_OK(decimalBuilder.Finish(&decimalArray));
 
   arrow::StringBuilder strbuilder;
   PARQUET_THROW_NOT_OK(strbuilder.Append("some"));
@@ -42,9 +44,9 @@ std::shared_ptr<arrow::Table> generate_table() {
   PARQUET_THROW_NOT_OK(strbuilder.Finish(&strarray));
 
   std::shared_ptr<arrow::Schema> schema = arrow::schema(
-      {arrow::field("int", arrow::int64()), arrow::field("str", arrow::utf8())});
+      {arrow::field("decimal", arrow::decimal(15, 15)), arrow::field("str", arrow::utf8())});
 
-  return arrow::Table::Make(schema, {i64array, strarray});
+  return arrow::Table::Make(schema, {decimalArray, strarray});
 }
 
 // #1 Write out the data as a Parquet file

I've do an experiment with C++ arrow parquet, and it's able to write decimal with Decimal(15, 15). So I think this patch is ok

[wgtmac]

LGTM. Thanks @devinrsmith and @mapleFU !