presto-hive/src/main/java/io/prestosql/plugin/hive/HivePageSourceProvider.java/HivePageSourceProvider/prefilled
class HivePageSourceProvider:        public static ColumnMapping prefilled(HiveColumnHandle hiveColumnHandle, String prefilledValue, Optional<HiveType> coerceFrom)
        {
            checkArgument(hiveColumnHandle.getColumnType() == PARTITION_KEY || hiveColumnHandle.getColumnType() == SYNTHESIZED);
            return new ColumnMapping(ColumnMappingKind.PREFILLED, hiveColumnHandle, Optional.of(prefilledValue), OptionalInt.empty(), coerceFrom);
        }