prestodb/presto · error · IllegalArgumentException

Unsupported flat map key type:

Error message

Unsupported flat map key type: 

What it means

MapFlatSelectiveStreamReader creates the key block for flat-map (DWRF_MAP_FLAT) columns. Only INTEGER and LONG key kinds (and STRING/BINARY via getSliceKeysBlock) are supported; any other key ORC type kind throws IllegalArgumentException. This is a limitation of the selective flat-map reader's supported key types.

Source

Thrown at presto-orc/src/main/java/com/facebook/presto/orc/reader/MapFlatSelectiveStreamReader.java:711

            return requiredLongKeys.isEmpty() || requiredLongKeys.contains(value.getKey().getIntKey());
        }

        return requiredStringKeys.isEmpty() || requiredStringKeys.contains(value.getKey().getBytesKey().toStringUtf8());
    }

    private Block getKeysBlock(List<DwrfSequenceEncoding> sequenceEncodings)
    {
        switch (keyOrcTypeKind) {
            case BYTE:
            case SHORT:
            case INT:
            case LONG:
                return getIntegerKeysBlock(sequenceEncodings);
            case STRING:
            case BINARY:
                return getSliceKeysBlock(sequenceEncodings);
            default:
                throw new IllegalArgumentException("Unsupported flat map key type: " + keyOrcTypeKind);
        }
    }

    private Block getIntegerKeysBlock(List<DwrfSequenceEncoding> sequenceEncodings)
    {
        Type keyType;

        switch (keyOrcTypeKind) {
            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. Use a flat-map key ORC type among INTEGER, LONG, STRING, or BINARY
2. Re-encode the file with a supported key type
3. Use the non-selective (batch) reader or a non-flat map encoding
4. Verify the ORC/DWRF writer settings used to produce the file

Example fix

// before
throw new IllegalArgumentException("Unsupported flat map key type: " + keyOrcTypeKind);
// after
if (!SUPPORTED_FLAT_KEY_KINDS.contains(keyOrcTypeKind)) { throw new OrcCorruptionException(id, "Unsupported flat map key type: " + keyOrcTypeKind); }

Set<OrcTypeKind> ok = EnumSet.of(INTEGER, LONG, STRING, BINARY); if (!ok.contains(keyOrcTypeKind)) { throw new IllegalArgumentException("Unsupported flat map key: " + keyOrcTypeKind); }

boolean supportsFlatMapKey(OrcTypeKind t) { return t == INTEGER || t == LONG || t == STRING || t == BINARY; }

try { reader = createReader(descriptor, ...); } catch (IllegalArgumentException e) { if (e.getMessage().contains("Unsupported flat map key")) { fallbackToBatchReader(descriptor); } else throw e; }

When it happens

Trigger: Constructing MapFlatSelectiveStreamReader for a flat map column whose key type kind is not INTEGER, LONG, STRING, or BINARY (e.g., BYTE, SHORT, VARCHAR variants not mapped, FLOAT, structural kinds).

Common situations: Files written with flat map keys of uncommon types; writer/reader version skew; falling into the selective path for maps expected to have simple keys.

Related errors

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

AI-assisted analysis of prestodb/presto@55bb57d202 (2026-09-04). Data as JSON: /api/errors/c23453926396dbbc. Report an issue: GitHub.