apache/iceberg · error · UnsupportedOperationException

Unsupported base type for decimal: {primitive.getPrimitiveTy

Error message

Unsupported base type for decimal: {primitive.getPrimitiveTypeName()}

What it means

In SparkParquetReaders.primitive, decimal columns must be backed by FLBA (fixed_len_byte_array), INT64, or INT32 Parquet storage. If the underlying physical type of a decimal logical type is anything else, this UnsupportedOperationException is thrown because no decimal reader exists for that base type.

Source

Thrown at spark/v4.1/spark/src/main/java/org/apache/iceberg/spark/data/SparkParquetReaders.java:270

          case DATE:
          case INT_64:
            return new UnboxedReader<>(desc);
          case TIMESTAMP_MICROS:
          case TIMESTAMP_MILLIS:
            return ParquetValueReaders.timestamps(desc);
          case DECIMAL:
            DecimalLogicalTypeAnnotation decimal =
                (DecimalLogicalTypeAnnotation) primitive.getLogicalTypeAnnotation();
            switch (primitive.getPrimitiveTypeName()) {
              case BINARY:
              case FIXED_LEN_BYTE_ARRAY:
                return new BinaryDecimalReader(desc, decimal.getScale());
              case INT64:
                return new LongDecimalReader(desc, decimal.getPrecision(), decimal.getScale());
              case INT32:
                return new IntegerDecimalReader(desc, decimal.getPrecision(), decimal.getScale());
              default:
                throw new UnsupportedOperationException(
                    "Unsupported base type for decimal: " + primitive.getPrimitiveTypeName());
            }
          case BSON:
            return new ParquetValueReaders.ByteArrayReader(desc);
          default:
            throw new UnsupportedOperationException(
                "Unsupported logical type: " + primitive.getOriginalType());
        }
      }

      switch (primitive.getPrimitiveTypeName()) {
        case FIXED_LEN_BYTE_ARRAY:
        case BINARY:
          if (expected != null && expected.typeId() == TypeID.UUID) {
            return new UUIDReader(desc);
          }
          return new ParquetValueReaders.ByteArrayReader(desc);
        case INT32:

View on GitHub (pinned to 86d9c8fc54)

Solutions

1. Rewrite the source Parquet files with a writer that uses FLBA/INT64/INT32 storage for decimals (e.g. rewrite_data_files).
2. Verify the file's decimal encoding with parquet-tools and, if BINARY, convert the column (e.g. via Spark cast) before reading through Iceberg.
3. Upgrade Iceberg — newer versions may add support for more decimal physical representations.

// inspect physical encoding before reading
// parquet-tools schema file.parquet | grep -B1 DECIMAL

try {
  icebergTable.newScan().planFiles();
} catch (UnsupportedOperationException e) {
  if (e.getMessage().contains("Unsupported base type for decimal")) {
    // fall back to non-Iceberg read or rewrite the files
  }
}

When it happens

Trigger: Reading a Parquet file whose column has the DECIMAL logical type but a physical primitive type other than FIXED_LEN_BYTE_ARRAY, INT64, or INT32 (e.g. BINARY-backed decimal from some writers).

Common situations: Reading Parquet files produced by non-Iceberg writers (Hive, Impala, third-party tools) that store decimals in unusual physical encodings; mixed-writer tables.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Unsupported base type for decimal:
• Unsupported base type for decimal: ${primitive.getPrimitiveT
• Unsupported base type for decimal: ${primitiveTypeName}
• Unsupported base type for decimal: ${desc.getPrimitiveType()
• Unsupported base type for decimal: %s

AI-assisted analysis of apache/iceberg@86d9c8fc54 (2026-09-12). Data as JSON: /api/errors/3f0915b7d4c4e887. Report an issue: GitHub.