20.10.1
```
cat x.sql

drop table if exists enum_test;

create table enum_test(timestamp DateTime, host String, e Enum8('IU' = 1, 'WS' = 2))
Engine = MergeTree PARTITION BY toDate(timestamp) ORDER BY (timestamp, host);

insert into enum_test select '2020-10-09 00:00:00', 'h1', 'WS' from numbers(1000);
alter table enum_test modify column e Enum8('IU' = 1, 'WS' = 2, 'PS' = 3);
insert into enum_test select  '2020-10-09 00:00:00', 'h1', 'PS' from numbers(10);

SELECT * FROM enum_test ORDER BY timestamp DESC LIMIT 1;

Received exception from server (version 20.10.1):
Code: 49. DB::Exception: Received from localhost:9000. DB::Exception: Block structure mismatch in Pipe::unitePipes stream: different types:
timestamp DateTime UInt32(size = 0), host String String(size = 0), e Enum8('IU' = 1, 'WS' = 2, 'PS' = 3) Int8(size = 0)
timestamp DateTime UInt32(size = 0), host String String(size = 0), e Enum8('IU' = 1, 'WS' = 2) Int8(size = 0).


optimize table enum_test final;
SELECT timestamp, host FROM enum_test ORDER BY timestamp DESC LIMIT 1;
┌───────────timestamp─┬─host─┐
│ 2020-10-09 00:00:00 │ h1   │
└─────────────────────┴──────┘
```
{
    /** Close the files (before destroying the object).
    * When many sources are created, but simultaneously reading only a few of them,
    * buffers don't waste memory.
    */
    reader.reset();
    pre_reader.reset();
    data_part.reset();
}
MergeTreeReverseSelectProcessor::~MergeTreeReverseSelectProcessor() = default;
}