prestodb/presto · error · RuntimeException

Unknown object inspector category:

Error message

Unknown object inspector category: 

What it means

SerDeUtils.serializeObject switches on the Hive ObjectInspector category (PRIMITIVE, LIST, MAP, STRUCT). If the inspector's category is anything else (e.g. UNION), there is no serialization branch and it throws RuntimeException('Unknown object inspector category: ...'). The serializer only supports the standard Hive column categories.

Source

Thrown at presto-hive/src/main/java/com/facebook/presto/hive/util/SerDeUtils.java:101

    }

    // This version supports optionally disabling the filtering of null map key, which should only be used for building test data sets
    // that contain null map keys.  For production, null map keys are not allowed.
    @VisibleForTesting
    public static Block serializeObject(Type type, BlockBuilder builder, Object object, ObjectInspector inspector, boolean filterNullMapKeys, DateTimeZone hiveStorageTimeZone, boolean legacyTimestampEnabled)
    {
        switch (inspector.getCategory()) {
            case PRIMITIVE:
                serializePrimitive(type, builder, object, (PrimitiveObjectInspector) inspector, hiveStorageTimeZone, legacyTimestampEnabled);
                return null;
            case LIST:
                return serializeList(type, builder, object, (ListObjectInspector) inspector, hiveStorageTimeZone, legacyTimestampEnabled);
            case MAP:
                return serializeMap(type, builder, object, (MapObjectInspector) inspector, filterNullMapKeys, hiveStorageTimeZone, legacyTimestampEnabled);
            case STRUCT:
                return serializeStruct(type, builder, object, (StructObjectInspector) inspector, hiveStorageTimeZone, legacyTimestampEnabled);
        }
        throw new RuntimeException("Unknown object inspector category: " + inspector.getCategory());
    }

    private static void serializePrimitive(Type type, BlockBuilder builder, Object object, PrimitiveObjectInspector inspector, DateTimeZone hiveStorageTimeZone, boolean legacyTimestampEnabled)
    {
        requireNonNull(builder, "parent builder is null");

        if (object == null) {
            builder.appendNull();
            return;
        }

        switch (inspector.getPrimitiveCategory()) {
            case BOOLEAN:
                BooleanType.BOOLEAN.writeBoolean(builder, ((BooleanObjectInspector) inspector).get(object));
                return;
            case BYTE:
                TinyintType.TINYINT.writeLong(builder, ((ByteObjectInspector) inspector).get(object));
                return;

View on GitHub (pinned to 55bb57d202)

Solutions

1. Remove or flatten uniontype columns (store as string/struct instead)
2. Cast the column in the Hive view/query to a supported type before Presto reads it
3. Recreate the table with a supported schema (no UNION types)
4. Check whether the SerDe is producing an unexpected inspector for a normal type and fix the table properties

Example fix

// before (Hive)
CREATE TABLE t (u uniontype<int,string>);
// after
CREATE TABLE t (u string); -- or struct<a:int,b:string>

// verify category is supported before serializing
ObjectInspector.Category c = inspector.getCategory();
if (c != Category.PRIMITIVE && c != Category.LIST && c != Category.MAP && c != Category.STRUCT) {
    throw new PrestoException(NOT_SUPPORTED, "Unsupported inspector category: " + c);
}

boolean isSupportedCategory(ObjectInspector inspector) {
    switch (inspector.getCategory()) {
        case PRIMITIVE: case LIST: case MAP: case STRUCT: return true;
        default: return false;
    }
}

try {
    block = SerDeUtils.serializeObject(...);
} catch (RuntimeException e) {
    if (e.getMessage() != null && e.getMessage().startsWith("Unknown object inspector category")) {
        LOG.error("Column uses unsupported Hive type (e.g. uniontype): " + e.getMessage());
    }
    throw e;
}

When it happens

Trigger: Writing a Hive column whose ObjectInspector category is UNION or another non-standard category, recursively from serializeObject on a nested union field or a top-level union-typed column.

Common situations: Hive tables containing uniontype columns; SerDe returning exotic inspectors for certain formats; schema drift after altering a column to a union type.

Related errors

• Unknown primitive type:
• Unexpected column type
• unsupported string field type:
• Unsupported column type:
• unknown java type

AI-assisted analysis of prestodb/presto@55bb57d202 (2026-09-04). Data as JSON: /api/errors/be2040ef4499578f. Report an issue: GitHub.