create table aa (a DateTime, b DateTime, s String)
engine = MergeTree partition by toYYYYMMDD(b) order by (toStartOfHour(a), toStartOfHour(b));

insert into aa select toDateTime('2020-01-01 00:00:00')+number,
                      toDateTime('2020-02-01 00:00:00')-number, toString(number)
from numbers(1000000000);

select count() from aa where b = toDateTime('2020-01-01 15:00:00')

1 rows in set. Elapsed: 0.012 sec. Processed 86.40 thousand rows, 345.60 KB

select count() from aa where a = toDateTime('2020-01-01 15:00:00')
1 rows in set. Elapsed: 0.156 sec. Processed 16.38 thousand rows, 65.54 KB


без индекса
select count() from aa where identity(b) = toDateTime('2020-01-01 15:00:00')
1 rows in set. Elapsed: 0.502 sec. Processed 1.00 billion rows, 4.00 GB

2020.09.29 23:25:29.773636 [ 22715 ] {e38379aa-9d32-4528-8300-34e45fd55b4f} <Error> executeQuery: Code: 517, e.displayText() = DB::Exception: Metadata on replica is not up to date with common metadata in Zookeeper. Cannot
alter (version 20.6.6.7 (official build)) (from 10.7.224.76:60008) (in query: ALTER TABLE local.user_auth_events_local ADD COLUMN `unauth_id` Int64 AFTER user_id ), Stack trace (when copying this message, always include th
e lines below):

0. Poco::Exception::Exception(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, int) @ 0x123fea60 in /usr/bin/clickhouse
1. DB::Exception::Exception(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, int) @ 0xa23e3fd in /usr/bin/clickhouse
2. ? @ 0xf74b061 in /usr/bin/clickhouse
3. DB::InterpreterAlterQuery::execute() @ 0xf177ec1 in /usr/bin/clickhouse
4. ? @ 0xf4ddf0f in /usr/bin/clickhouse
5. DB::executeQuery(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, DB::Context&, bool, DB::QueryProcessingStage::Enum, bool) @ 0xf4e1675 in /usr/bin/clickhouse
6. DB::TCPHandler::runImpl() @ 0xfb5bb7c in /usr/bin/clickhouse
7. DB::TCPHandler::run() @ 0xfb5c900 in /usr/bin/clickhouse
8. Poco::Net::TCPServerConnection::start() @ 0x1231c88b in /usr/bin/clickhouse
9. Poco::Net::TCPServerDispatcher::run() @ 0x1231cd1b in /usr/bin/clickhouse
10. Poco::PooledThread::run() @ 0x1249b846 in /usr/bin/clickhouse
11. Poco::ThreadImpl::runnableEntry(void*) @ 0x12496c40 in /usr/bin/clickhouse
12. start_thread @ 0x74a4 in /lib/x86_64-linux-gnu/libpthread-2.24.so
13. __clone @ 0xe8d0f in /lib/x86_64-linux-gnu/libc-2.24.so