2021 Changelog

#### Experimental Feature[​](#experimental-feature-3 "Direct link to Experimental Feature")

- Added a compression codec `AES_128_GCM_SIV` which encrypts columns instead of compressing them. [#19896](https://github.com/ClickHouse/ClickHouse/pull/19896) ([PHO](https://github.com/depressed-pho)). Will be rewritten, do not use.
- Rename `MaterializeMySQL` to `MaterializedMySQL`. [#26822](https://github.com/ClickHouse/ClickHouse/pull/26822) ([tavplubix](https://github.com/tavplubix)).

#### Performance Improvement[​](#performance-improvement-3 "Direct link to Performance Improvement")

- Improve the performance of fast queries when `max_execution_time = 0` by reducing the number of `clock_gettime` system calls. [#27325](https://github.com/ClickHouse/ClickHouse/pull/27325) ([filimonov](https://github.com/filimonov)).
- Specialize date time related comparison to achieve better performance. This fixes [#27083](https://github.com/ClickHouse/ClickHouse/issues/27083) . [#27122](https://github.com/ClickHouse/ClickHouse/pull/27122) ([Amos Bird](https://github.com/amosbird)).
- Share file descriptors in concurrent reads of the same files. There is no noticeable performance difference on Linux. But the number of opened files will be significantly (10..100 times) lower on typical servers and it makes operations easier. See [#26214](https://github.com/ClickHouse/ClickHouse/issues/26214). [#26768](https://github.com/ClickHouse/ClickHouse/pull/26768) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Improve latency of short queries, that require reading from tables with large number of columns. [#26371](https://github.com/ClickHouse/ClickHouse/pull/26371) ([Anton Popov](https://github.com/CurtizJ)).
- Don't build sets for indices when analyzing a query. [#26365](https://github.com/ClickHouse/ClickHouse/pull/26365) ([Raúl Marín](https://github.com/Algunenano)).
- Vectorize the SUM of Nullable integer types with native representation ([David Manzanares](https://github.com/davidmanzanares), [Raúl Marín](https://github.com/Algunenano)). [#26248](https://github.com/ClickHouse/ClickHouse/pull/26248) ([Raúl Marín](https://github.com/Algunenano)).
- Compile expressions involving columns with `Enum` types. [#26237](https://github.com/ClickHouse/ClickHouse/pull/26237) ([Maksim Kita](https://github.com/kitaisreal)).
- Compile aggregate functions `groupBitOr`, `groupBitAnd`, `groupBitXor`. [#26161](https://github.com/ClickHouse/ClickHouse/pull/26161) ([Maksim Kita](https://github.com/kitaisreal)).
- Improved memory usage with better block size prediction when reading empty DEFAULT columns. Closes [#17317](https://github.com/ClickHouse/ClickHouse/issues/17317). [#25917](https://github.com/ClickHouse/ClickHouse/pull/25917) ([Vladimir Chebotarev](https://github.com/excitoon)).
- Reduce memory usage and number of read rows in queries with `ORDER BY primary_key`. [#25721](https://github.com/ClickHouse/ClickHouse/pull/25721) ([Anton Popov](https://github.com/CurtizJ)).
- Enable `distributed_push_down_limit` by default. [#27104](https://github.com/ClickHouse/ClickHouse/pull/27104) ([Azat Khuzhin](https://github.com/azat)).
- Make `toTimeZone` monotonicity when timeZone is a constant value to support partition puring when use sql like:. [#26261](https://github.com/ClickHouse/ClickHouse/pull/26261) ([huangzhaowei](https://github.com/SaintBacchus)).

#### Improvement[​](#improvement-3 "Direct link to Improvement")