apache/iceberg · error · UnsupportedOperationException

Unsupported base type for decimal:

Error message

Unsupported base type for decimal: 

What it means

SparkParquetReaders.primitive() reads DECIMAL logical types from Parquet. Decimals must be physically stored as INT32, INT64, or FIXED_LEN_BYTE_ARRAY. If the underlying Parquet primitive is anything else (e.g. BINARY or FLOAT), the reader throws UnsupportedOperationException because it cannot decode a decimal from that storage type.

Source

Thrown at spark/v4.0/spark/src/main/java/org/apache/iceberg/spark/data/SparkParquetReaders.java:257

          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. Inspect the Parquet file schema (parquet-tools / parquet cat --schema) and confirm the decimal column's physical type
2. Rewrite the offending data files with a standard producer (Spark/Iceberg) so decimals use INT32/INT64/FIXED_LEN_BYTE_ARRAY
3. Re-annotate or re-encode the column as BINARY if the data is not actually decimal, and update the table schema accordingly
4. Upgrade Iceberg if support for additional decimal encodings has been added in newer versions

MessageType schema = parquetFileReader.getFooter().getFileMetaData().getSchema();
for (ColumnDescriptor col : schema.getColumns()) {
  if (col.getPrimitiveTypeName() == PrimitiveTypeName.BINARY
      && schema.containsPath(col.getPath()) /* check annotation */ ) {
    // verify decimal columns use INT32/INT64/FIXED_LEN_BYTE_ARRAY before reading
  }
}

try {
  spark.read().format("iceberg").load("db.tbl");
} catch (UnsupportedOperationException e) {
  if (e.getMessage().startsWith("Unsupported base type for decimal")) {
    // rewrite the offending files with a standard Parquet producer
  } else {
    throw e;
  }
}

When it happens

Trigger: Reading a Parquet file whose column is annotated as DECIMAL logical type but whose physical type is not INT32/INT64/FIXED_LEN_BYTE_ARRAY — encountered while doing a Spark read of an Iceberg Parquet table.

Common situations: Files written by non-standard Parquet producers that store decimals in BINARY without the expected FLBA layout; corrupted schema metadata; third-party tools that re-annotated columns as DECIMAL without re-encoding the physical data.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

• Unsupported base type for decimal:
• Unsupported base type for decimal: ${primitive.getPrimitiveT
• Unsupported base type for decimal: ${primitive.getPrimitiveT
• Unsupported type: ${primitive}
• Unsupported logical type:

AI-assisted analysis of apache/iceberg@86d9c8fc54 (2026-09-12). Data as JSON: /api/errors/1792506cc5f31eb7. Report an issue: GitHub.