presto-hive/src/main/java/io/prestosql/plugin/hive/HivePageSource.java/HivePageSource/MapCoercer
class HivePageSource:        public MapCoercer(TypeManager typeManager, HiveType fromHiveType, HiveType toHiveType)
        {
            requireNonNull(typeManager, "typeManage is null");
            requireNonNull(fromHiveType, "fromHiveType is null");
            this.toType = requireNonNull(toHiveType, "toHiveType is null").getType(typeManager);
            HiveType fromKeyHiveType = HiveType.valueOf(((MapTypeInfo) fromHiveType.getTypeInfo()).getMapKeyTypeInfo().getTypeName());
            HiveType fromValueHiveType = HiveType.valueOf(((MapTypeInfo) fromHiveType.getTypeInfo()).getMapValueTypeInfo().getTypeName());
            HiveType toKeyHiveType = HiveType.valueOf(((MapTypeInfo) toHiveType.getTypeInfo()).getMapKeyTypeInfo().getTypeName());
            HiveType toValueHiveType = HiveType.valueOf(((MapTypeInfo) toHiveType.getTypeInfo()).getMapValueTypeInfo().getTypeName());
            this.keyCoercer = fromKeyHiveType.equals(toKeyHiveType) ? null : createCoercer(typeManager, fromKeyHiveType, toKeyHiveType);
            this.valueCoercer = fromValueHiveType.equals(toValueHiveType) ? null : createCoercer(typeManager, fromValueHiveType, toValueHiveType);
        }