2020 Changelog

#### Performance Improvement[​](#performance-improvement-1 "Direct link to Performance Improvement")

- Do not merge parts across partitions in SELECT FINAL. [#15938](https://github.com/ClickHouse/ClickHouse/pull/15938) ([Kruglov Pavel](https://github.com/Avogar)).
- Improve performance of `-OrNull` and `-OrDefault` aggregate functions. [#16661](https://github.com/ClickHouse/ClickHouse/pull/16661) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Improve performance of `quantileMerge`. In previous versions it was obnoxiously slow. This closes [#1463](https://github.com/ClickHouse/ClickHouse/issues/1463). [#16643](https://github.com/ClickHouse/ClickHouse/pull/16643) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Improve performance of logical functions a little. [#16347](https://github.com/ClickHouse/ClickHouse/pull/16347) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Improved performance of merges assignment in MergeTree table engines. Shouldn't be visible for the user. [#16191](https://github.com/ClickHouse/ClickHouse/pull/16191) ([alesapin](https://github.com/alesapin)).
- Speedup hashed/sparse\_hashed dictionary loading by preallocating the hash table. [#15454](https://github.com/ClickHouse/ClickHouse/pull/15454) ([Azat Khuzhin](https://github.com/azat)).
- Now trivial count optimization becomes slightly non-trivial. Predicates that contain exact partition expr can be optimized too. This also fixes [#11092](https://github.com/ClickHouse/ClickHouse/issues/11092) which returns wrong count when `max_parallel_replicas > 1`. [#15074](https://github.com/ClickHouse/ClickHouse/pull/15074) ([Amos Bird](https://github.com/amosbird)).

#### Build/Testing/Packaging Improvement[​](#buildtestingpackaging-improvement-3 "Direct link to Build/Testing/Packaging Improvement")

- Add flaky check for stateless tests. It will detect potentially flaky functional tests in advance, before they are merged. [#16238](https://github.com/ClickHouse/ClickHouse/pull/16238) ([alesapin](https://github.com/alesapin)).
- Use proper version for `croaring` instead of amalgamation. [#16285](https://github.com/ClickHouse/ClickHouse/pull/16285) ([sundyli](https://github.com/sundy-li)).
- Improve generation of build files for `ya.make` build system (Arcadia). [#16700](https://github.com/ClickHouse/ClickHouse/pull/16700) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Add MySQL BinLog file check tool for `MaterializeMySQL` database engine. `MaterializeMySQL` is an experimental feature. [#16223](https://github.com/ClickHouse/ClickHouse/pull/16223) ([Winter Zhang](https://github.com/zhang2014)).
- Check for executable bit on non-executable files. People often accidentially commit executable files from Windows. [#15843](https://github.com/ClickHouse/ClickHouse/pull/15843) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Check for `#pragma once` in headers. [#15818](https://github.com/ClickHouse/ClickHouse/pull/15818) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Fix illegal code style `&vector[idx]` in libhdfs3. This fixes libcxx debug build. See also <https://github.com/ClickHouse-Extras/libhdfs3/pull/8> . [#15815](https://github.com/ClickHouse/ClickHouse/pull/15815) ([Amos Bird](https://github.com/amosbird)).
- Fix build of one miscellaneous example tool on Mac OS. Note that we don't build examples on Mac OS in our CI (we build only ClickHouse binary), so there is zero chance it will not break again. This fixes [#15804](https://github.com/ClickHouse/ClickHouse/issues/15804). [#15808](https://github.com/ClickHouse/ClickHouse/pull/15808) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Simplify Sys/V init script. [#14135](https://github.com/ClickHouse/ClickHouse/pull/14135) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Added `boost::program_options` to `db_generator` in order to increase its usability. This closes [#15940](https://github.com/ClickHouse/ClickHouse/issues/15940). [#15973](https://github.com/ClickHouse/ClickHouse/pull/15973) ([Nikita Mikhaylov](https://github.com/nikitamikhaylov)).

## ClickHouse release 20.10[​](#clickhouse-release-2010 "Direct link to ClickHouse release 20.10")

### ClickHouse release v20.10.7.4-stable, 2020-12-24[​](#clickhouse-release-v201074-stable-2020-12-24 "Direct link to ClickHouse release v20.10.7.4-stable, 2020-12-24")

#### Bug Fix[​](#bug-fix-7 "Direct link to Bug Fix")