2020 Changelog

#### Build/Testing/Packaging Improvement[​](#buildtestingpackaging-improvement-4 "Direct link to Build/Testing/Packaging Improvement")

- Now we use clang-11 for production ClickHouse build. [#15239](https://github.com/ClickHouse/ClickHouse/pull/15239) ([alesapin](https://github.com/alesapin)).
- Now we use clang-11 to build ClickHouse in CI. [#14846](https://github.com/ClickHouse/ClickHouse/pull/14846) ([alesapin](https://github.com/alesapin)).
- Switch binary builds (Linux, Darwin, AArch64, FreeDSD) to clang-11. [#15622](https://github.com/ClickHouse/ClickHouse/pull/15622) ([Ilya Yatsishin](https://github.com/qoega)).
- Now all test images use `llvm-symbolizer-11`. [#15069](https://github.com/ClickHouse/ClickHouse/pull/15069) ([alesapin](https://github.com/alesapin)).
- Allow to build with llvm-11. [#15366](https://github.com/ClickHouse/ClickHouse/pull/15366) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Switch from `clang-tidy-10` to `clang-tidy-11`. [#14922](https://github.com/ClickHouse/ClickHouse/pull/14922) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Use LLVM's experimental pass manager by default. [#15608](https://github.com/ClickHouse/ClickHouse/pull/15608) ([Danila Kutenin](https://github.com/danlark1)).
- Don't allow any C++ translation unit to build more than 10 minutes or to use more than 10 GB or memory. This fixes [#14925](https://github.com/ClickHouse/ClickHouse/issues/14925). [#15060](https://github.com/ClickHouse/ClickHouse/pull/15060) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Make performance test more stable and representative by splitting test runs and profile runs. [#15027](https://github.com/ClickHouse/ClickHouse/pull/15027) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Attempt to make performance test more reliable. It is done by remapping the executable memory of the process on the fly with `madvise` to use transparent huge pages - it can lower the number of iTLB misses which is the main source of instabilities in performance tests. [#14685](https://github.com/ClickHouse/ClickHouse/pull/14685) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Convert to python3. This closes [#14886](https://github.com/ClickHouse/ClickHouse/issues/14886). [#15007](https://github.com/ClickHouse/ClickHouse/pull/15007) ([Azat Khuzhin](https://github.com/azat)).
- Fail early in functional tests if server failed to respond. This closes [#15262](https://github.com/ClickHouse/ClickHouse/issues/15262). [#15267](https://github.com/ClickHouse/ClickHouse/pull/15267) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Allow to run AArch64 version of clickhouse-server without configs. This facilitates [#15174](https://github.com/ClickHouse/ClickHouse/issues/15174). [#15266](https://github.com/ClickHouse/ClickHouse/pull/15266) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Improvements in CI docker images: get rid of ZooKeeper and single script for test configs installation. [#15215](https://github.com/ClickHouse/ClickHouse/pull/15215) ([alesapin](https://github.com/alesapin)).
- Fix CMake options forwarding in fast test script. Fixes error in [#14711](https://github.com/ClickHouse/ClickHouse/issues/14711). [#15155](https://github.com/ClickHouse/ClickHouse/pull/15155) ([alesapin](https://github.com/alesapin)).
- Added a script to perform hardware benchmark in a single command. [#15115](https://github.com/ClickHouse/ClickHouse/pull/15115) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Splitted huge test `test_dictionaries_all_layouts_and_sources` into smaller ones. [#15110](https://github.com/ClickHouse/ClickHouse/pull/15110) ([Nikita Mikhaylov](https://github.com/nikitamikhaylov)).
- Maybe fix MSan report in base64 (on servers with AVX-512). This fixes [#14006](https://github.com/ClickHouse/ClickHouse/issues/14006). [#15030](https://github.com/ClickHouse/ClickHouse/pull/15030) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Reformat and cleanup code in all integration test \*.py files. [#14864](https://github.com/ClickHouse/ClickHouse/pull/14864) ([Bharat Nallan](https://github.com/bharatnc)).
- Fix MaterializeMySQL empty transaction unstable test case found in CI. [#14854](https://github.com/ClickHouse/ClickHouse/pull/14854) ([Winter Zhang](https://github.com/zhang2014)).
- Attempt to speed up build a little. [#14808](https://github.com/ClickHouse/ClickHouse/pull/14808) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Speed up build a little by removing unused headers. [#14714](https://github.com/ClickHouse/ClickHouse/pull/14714) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Fix build failure in OSX. [#14761](https://github.com/ClickHouse/ClickHouse/pull/14761) ([Winter Zhang](https://github.com/zhang2014)).
- Enable ccache by default in cmake if it's found in OS. [#14575](https://github.com/ClickHouse/ClickHouse/pull/14575) ([alesapin](https://github.com/alesapin)).
- Control CI builds configuration from the ClickHouse repository. [#14547](https://github.com/ClickHouse/ClickHouse/pull/14547) ([alesapin](https://github.com/alesapin)).
- In CMake files: - Moved some options' descriptions' parts to comments above. - Replace 0 -> `OFF`, 1 -> `ON` in `option`s default values. - Added some descriptions and links to docs to the options. - Replaced `FUZZER` option (there is another option `ENABLE_FUZZING` which also enables same functionality). - Removed `ENABLE_GTEST_LIBRARY` option as there is `ENABLE_TESTS`. See the full description in PR: [#14711](https://github.com/ClickHouse/ClickHouse/pull/14711) ([Mike](https://github.com/myrrc)).
- Make binary a bit smaller (~50 Mb for debug version). [#14555](https://github.com/ClickHouse/ClickHouse/pull/14555) ([Artem Zuikov](https://github.com/4ertus2)).
- Use std::filesystem::path in ConfigProcessor for concatenating file paths. [#14558](https://github.com/ClickHouse/ClickHouse/pull/14558) ([Bharat Nallan](https://github.com/bharatnc)).
- Fix debug assertion in `bitShiftLeft()` when called with negative big integer. [#14697](https://github.com/ClickHouse/ClickHouse/pull/14697) ([Artem Zuikov](https://github.com/4ertus2)).

## ClickHouse release 20.9[​](#clickhouse-release-209 "Direct link to ClickHouse release 20.9")

### ClickHouse release v20.9.7.11-stable, 2020-12-07[​](#clickhouse-release-v209711-stable-2020-12-07 "Direct link to ClickHouse release v20.9.7.11-stable, 2020-12-07")

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

- Fix performance of reading from `Merge` tables over huge number of `MergeTree` tables. Fixes [#7748](https://github.com/ClickHouse/ClickHouse/issues/7748). [#16988](https://github.com/ClickHouse/ClickHouse/pull/16988) ([Anton Popov](https://github.com/CurtizJ)).

#### Bug Fix[​](#bug-fix-10 "Direct link to Bug Fix")