prestodb/presto · error · OrcCorruptionException

Flat map reader does not support nesting

Error message

Flat map reader does not support nesting 

What it means

MapSelectiveStreamReader.startStripe dispatches to a direct or flat reader based on the stripe's column encoding. A DWRF_MAP_FLAT encoding requires a flat reader to have been created; if flatReader is null (e.g., flat map encountered in a nested context that wasn't set up), it throws OrcCorruptionException. Flat map readers cannot be instantiated for nested positions.

Source

Thrown at presto-orc/src/main/java/com/facebook/presto/orc/reader/MapSelectiveStreamReader.java:89

            // When sequence id is not DEFAULT_SEQUENCE_ID, this map is inside a flat map.
            // Flat maps can't be nested within another flat map.
            flatReader = null;
        }
    }

    @Override
    public void startStripe(ZoneId timezone, Stripe stripe)
            throws IOException
    {
        ColumnEncodingKind kind = stripe.getColumnEncodings().get(streamDescriptor.getStreamId())
                .getColumnEncoding(streamDescriptor.getSequence())
                .getColumnEncodingKind();
        if (kind == DIRECT || kind == DIRECT_V2 || kind == DWRF_DIRECT) {
            currentReader = directReader;
        }
        else if (kind == DWRF_MAP_FLAT) {
            if (flatReader == null) {
                throw new OrcCorruptionException(streamDescriptor.getOrcDataSourceId(), "Flat map reader does not support nesting " + streamDescriptor);
            }
            currentReader = flatReader;
        }
        else {
            throw new IllegalArgumentException("Unsupported encoding " + kind);
        }

        currentReader.startStripe(timezone, stripe);
    }

    @Override
    public void startRowGroup(InputStreamSources dataStreamSources)
            throws IOException
    {
        currentReader.startRowGroup(dataStreamSources);
    }

    @Override

View on GitHub (pinned to 55bb57d202)

Solutions

1. Rewrite the file so nested maps use DIRECT/DWRF_DIRECT encoding
2. Read the top-level map without the nested subfield pruning that creates nested readers
3. Upgrade Presto so flat-map nesting is supported or rejected earlier with a clearer message
4. Validate file encoding with a metadata scan before query

Example fix

// before: throw at runtime
if (flatReader == null) { throw new OrcCorruptionException(...); }
// after: reject at construction
checkState(encodingKind != DWRF_MAP_FLAT || supportsFlat, "DWRF_MAP_FLAT not supported for nested stream: %s", streamDescriptor);

if (encodingKind == ColumnEncodingKind.DWRF_MAP_FLAT && isNested) { throw new IllegalArgumentException("DWRF_MAP_FLAT is not supported for nested map columns"); }

null

try { reader.startStripe(timezone, stripe); } catch (OrcCorruptionException e) { if (e.getMessage().contains("Flat map reader does not support nesting")) { throw new UnsupportedFileFormatException(e); } throw e; }

When it happens

Trigger: A stripe whose map column encoding kind is DWRF_MAP_FLAT but the reader was constructed without a flat reader (flat maps not supported at nested/inner levels of a schema).

Common situations: Reading a file where flat-map encoding is applied to nested maps (map inside map/struct/array); writer emitted flat encoding where only direct is legal for nested columns.

Related errors

• Stripe encryption keys are missing, but file is encrypted
• Number of stripe encryption keys did not match number of enc
• Unsupported flat map key type:
• HIVE_INVALID_METADATA
• HIVE_CURSOR_ERROR

AI-assisted analysis of prestodb/presto@55bb57d202 (2026-09-04). Data as JSON: /api/errors/ed3b227e97c81215. Report an issue: GitHub.