prestodb/presto · error · IllegalArgumentException

Unsupported flat map key type:

Error message

Unsupported flat map key type: 

What it means

MapFlatBatchStreamReader.getKeyBlockTemplate builds a Block template for the flat map's key column based on the key's ORC type. Only LONG and STRING/BINARY keys are supported; any other keyOrcType (e.g. DOUBLE, TIMESTAMP, DECIMAL) hits the default branch and throws IllegalArgumentException('Unsupported flat map key type: ' + keyOrcType). DWRF flat map storage only defines inlined key storage for those types.

Source

Thrown at presto-orc/src/main/java/com/facebook/presto/orc/reader/MapFlatBatchStreamReader.java:285

        presentStream = null;

        rowGroupOpen = false;
    }

    private Block getKeyBlockTemplate(Collection<DwrfSequenceEncoding> sequenceEncodings)
    {
        switch (keyOrcType) {
            case BYTE:
            case SHORT:
            case INT:
            case LONG:
                return getIntegerKeyBlockTemplate(sequenceEncodings);
            case STRING:
            case BINARY:
                return getSliceKeysBlockTemplate(sequenceEncodings);
            default:
                throw new IllegalArgumentException("Unsupported flat map key type: " + keyOrcType);
        }
    }

    private Block getIntegerKeyBlockTemplate(Collection<DwrfSequenceEncoding> sequenceEncodings)
    {
        Type keyType;

        switch (keyOrcType) {
            case BYTE:
                keyType = TinyintType.TINYINT;
                break;
            case SHORT:
                keyType = SmallintType.SMALLINT;
                break;
            case INT:
                keyType = IntegerType.INTEGER;
                break;
            case LONG:

View on GitHub (pinned to 55bb57d202)

Solutions

1. Inspect the flat map's key type in the DWRF footer to confirm which type is used.
2. Upgrade Presto to a version supporting the key type (newer versions added more key templates).
3. Re-write the data as a regular MAP column (MAP_DIRECT encoding) instead of flat map.
4. Convert the map keys to STRING on the writer side so the STRING/BINARY template is used.

Example fix

// before — flat map keyed by DOUBLE
{"flatMap": {1.5: "a"}}
// after — cast keys to STRING on write
{"flatMap": {"1.5": "a"}}

OrcType keyOrcType = dwrfType.getKeyType();
if (!(keyOrcType.getOrcTypeKind() == LONG || keyOrcType.getOrcTypeKind() == STRING || keyOrcType.getOrcTypeKind() == BINARY)) {
    throw new PrestoException(NOT_SUPPORTED, "Flat map key type " + keyOrcType + " unsupported; rewrite as MAP");
}

boolean isSupportedFlatMapKeyType(OrcTypeKind kind) {
    return kind == LONG || kind == STRING || kind == BINARY;
}

try {
    reader.startStripe(timezone, stripe);
}
catch (IllegalArgumentException e) {
    throw new PrestoException(NOT_SUPPORTED, "Flat map key type not supported: " + e.getMessage(), e);
}

When it happens

Trigger: startStripe on a DWRF FLAT_MAP column whose key type is not LONG, STRING, or BINARY — e.g. flat maps keyed by double/decimal/timestamp written by DWRF tooling.

Common situations: Reading DWRF files that use extended flat-map key types unsupported by the current Presto version; writer/tooling emitting flat maps with non-standard key types; cross-version file interchange.

Related errors

• Unsupported flat map key type:
• Flat map reader does not support nesting
• Unsupported encoding
• NOT_SUPPORTED
• NOT_SUPPORTED

AI-assisted analysis of prestodb/presto@55bb57d202 (2026-09-04). Data as JSON: /api/errors/1b95b0813fe47f82. Report an issue: GitHub.