apache/iceberg · error · UnsupportedOperationException

Unsupported base type for decimal: %s

Error message

Unsupported base type for decimal: %s

What it means

In ParquetAvroWriter.primitive, a DECIMAL logical type whose underlying Parquet primitive is INT32/INT64 (non-BINARY/FIXED_LEN_BYTE_ARRAY) cannot be mapped to an Avro decimal writer. The library throws because the decimalAsFixed/decimalAsLong writers expect byte-backed storage. INT32/INT64-backed decimals are not supported by this Avro path.

Source

Thrown at parquet/src/main/java/org/apache/iceberg/parquet/ParquetAvroWriter.java:145

          case TIME_MICROS:
          case TIMESTAMP_MICROS:
            return ParquetValueWriters.longs(desc);
          case DECIMAL:
            DecimalLogicalTypeAnnotation decimal =
                (DecimalLogicalTypeAnnotation) primitive.getLogicalTypeAnnotation();
            switch (primitive.getPrimitiveTypeName()) {
              case INT32:
                return ParquetValueWriters.decimalAsInteger(
                    desc, decimal.getPrecision(), decimal.getScale());
              case INT64:
                return ParquetValueWriters.decimalAsLong(
                    desc, decimal.getPrecision(), decimal.getScale());
              case BINARY:
              case FIXED_LEN_BYTE_ARRAY:
                return ParquetValueWriters.decimalAsFixed(
                    desc, decimal.getPrecision(), decimal.getScale());
              default:
                throw new UnsupportedOperationException(
                    "Unsupported base type for decimal: " + primitive.getPrimitiveTypeName());
            }
          case BSON:
            return ParquetValueWriters.byteBuffers(desc);
          default:
            throw new UnsupportedOperationException(
                "Unsupported logical type: " + primitive.getOriginalType());
        }
      }

      switch (primitive.getPrimitiveTypeName()) {
        case FIXED_LEN_BYTE_ARRAY:
          return new FixedWriter(desc);
        case BINARY:
          return ParquetValueWriters.byteBuffers(desc);
        case BOOLEAN:
          return ParquetValueWriters.booleans(desc);
        case INT32:

View on GitHub (pinned to 86d9c8fc54)

Solutions

1. Rewrite the data so the decimal is stored as BINARY or FIXED_LEN_BYTE_ARRAY physical type
2. Use a writer that handles INT32/INT64-backed decimals (non-Avro Iceberg Parquet writer)
3. Convert the column to a string or double before writing via the Avro path

Example fix

// before
throw new UnsupportedOperationException("Unsupported base type for decimal: " + primitive.getPrimitiveTypeName());
// after (engine-side fix): force binary-backed decimal
Types.DecimalType.of(precision, scale) written via FIXED_LEN_BYTE_ARRAY storage

boolean badDecimal = schema.columns().stream().anyMatch(c ->
    c.type() instanceof Types.DecimalType && columnPhysical(c) != FIXED_LEN_BYTE_ARRAY && columnPhysical(c) != BINARY);
if (badDecimal) throw new IllegalArgumentException("Decimal must be BINARY/FIXED_LEN_BYTE_ARRAY backed");

try { writer.write(record); } catch (UnsupportedOperationException e) { if (e.getMessage().startsWith("Unsupported base type for decimal")) { /* rewrite decimal column as binary-backed */ } else { throw e; } }

When it happens

Trigger: Writing a Parquet column annotated DECIMAL with physical type INT32 or INT64 (or any primitive other than BINARY/FIXED_LEN_BYTE_ARRAY) through ParquetAvroWriter.primitive.

Common situations: Parquet files produced by other engines (e.g., Spark/Hive writing decimals as INT32/INT64 per newer spec revisions) are written via the Avro writer path; schema evolution changed decimal backing storage.

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: ${primitiveTypeName}
• Unsupported base type for decimal: ${desc.getPrimitiveType()
• Avro writer does not support variant types
• Unsupported logical type: %s
• Unsupported base type for decimal:

AI-assisted analysis of apache/iceberg@86d9c8fc54 (2026-09-12). Data as JSON: /api/errors/aa7a7d5ac3a88d06. Report an issue: GitHub.