presto-parquet/src/main/java/io/prestosql/parquet/reader/DecimalColumnReaderFactory.java/DecimalColumnReaderFactory/createReader
class DecimalColumnReaderFactory:    public static PrimitiveColumnReader createReader(RichColumnDescriptor descriptor, int precision, int scale)
    {
        DecimalType decimalType = DecimalType.createDecimalType(precision, scale);
        if (decimalType.isShort()) {
            return new ShortDecimalColumnReader(descriptor);
        }
        else {
            return new LongDecimalColumnReader(descriptor);
        }
    }