2020 Changelog

#### Experimental Feature[​](#experimental-feature-2 "Direct link to Experimental Feature")

- Added `db-generator` tool for random database generation by given SELECT queries. It may faciliate reproducing issues when there is only incomplete bug report from the user. [#14442](https://github.com/ClickHouse/ClickHouse/pull/14442) ([Nikita Mikhaylov](https://github.com/nikitamikhaylov)) [#10973](https://github.com/ClickHouse/ClickHouse/issues/10973) ([ZeDRoman](https://github.com/ZeDRoman)).

#### Improvement[​](#improvement-8 "Direct link to Improvement")

- Allow using multi-volume storage configuration in storage Distributed. [#14839](https://github.com/ClickHouse/ClickHouse/pull/14839) ([Pavel Kovalenko](https://github.com/Jokser)).
- Disallow empty time\_zone argument in `toStartOf*` type of functions. [#14509](https://github.com/ClickHouse/ClickHouse/pull/14509) ([Bharat Nallan](https://github.com/bharatnc)).
- MySQL handler returns `OK` for queries like `SET @@var = value`. Such statement is ignored. It is needed because some MySQL drivers send `SET @@` query for setup after handshake <https://github.com/ClickHouse/ClickHouse/issues/9336#issuecomment-686222422> . [#14469](https://github.com/ClickHouse/ClickHouse/pull/14469) ([BohuTANG](https://github.com/BohuTANG)).
- Now TTLs will be applied during merge if they were not previously materialized. [#14438](https://github.com/ClickHouse/ClickHouse/pull/14438) ([alesapin](https://github.com/alesapin)).
- Now `clickhouse-obfuscator` supports UUID type as proposed in [#13163](https://github.com/ClickHouse/ClickHouse/issues/13163). [#14409](https://github.com/ClickHouse/ClickHouse/pull/14409) ([dimarub2000](https://github.com/dimarub2000)).
- Added new setting `system_events_show_zero_values` as proposed in [#11384](https://github.com/ClickHouse/ClickHouse/issues/11384). [#14404](https://github.com/ClickHouse/ClickHouse/pull/14404) ([dimarub2000](https://github.com/dimarub2000)).
- Implicitly convert primary key to not null in `MaterializeMySQL` (Same as `MySQL`). Fixes [#14114](https://github.com/ClickHouse/ClickHouse/issues/14114). [#14397](https://github.com/ClickHouse/ClickHouse/pull/14397) ([Winter Zhang](https://github.com/zhang2014)).
- Replace wide integers (256 bit) from boost multiprecision with implementation from <https://github.com/cerevra/int>. 256bit integers are experimental. [#14229](https://github.com/ClickHouse/ClickHouse/pull/14229) ([Artem Zuikov](https://github.com/4ertus2)).
- Add default compression codec for parts in `system.part_log` with the name `default_compression_codec`. [#14116](https://github.com/ClickHouse/ClickHouse/pull/14116) ([alesapin](https://github.com/alesapin)).
- Add precision argument for `DateTime` type. It allows to use `DateTime` name instead of `DateTime64`. [#13761](https://github.com/ClickHouse/ClickHouse/pull/13761) ([Winter Zhang](https://github.com/zhang2014)).
- Added requirepass authorization for `Redis` external dictionary. [#13688](https://github.com/ClickHouse/ClickHouse/pull/13688) ([Ivan Torgashov](https://github.com/it1804)).
- Improvements in `RabbitMQ` engine: added connection and channels failure handling, proper commits, insert failures handling, better exchanges, queue durability and queue resume opportunity, new queue settings. Fixed tests. [#12761](https://github.com/ClickHouse/ClickHouse/pull/12761) ([Kseniia Sumarokova](https://github.com/kssenii)).
- Support custom codecs in compact parts. [#12183](https://github.com/ClickHouse/ClickHouse/pull/12183) ([Anton Popov](https://github.com/CurtizJ)).

#### Performance Improvement[​](#performance-improvement-4 "Direct link to Performance Improvement")

- Optimize queries with LIMIT/LIMIT BY/ORDER BY for distributed with GROUP BY sharding\_key (under `optimize_skip_unused_shards` and `optimize_distributed_group_by_sharding_key`). [#10373](https://github.com/ClickHouse/ClickHouse/pull/10373) ([Azat Khuzhin](https://github.com/azat)).
- Creating sets for multiple `JOIN` and `IN` in parallel. It may slightly improve performance for queries with several different `IN subquery` expressions. [#14412](https://github.com/ClickHouse/ClickHouse/pull/14412) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).
- Improve Kafka engine performance by providing independent thread for each consumer. Separate thread pool for streaming engines (like Kafka). [#13939](https://github.com/ClickHouse/ClickHouse/pull/13939) ([fastio](https://github.com/fastio)).

#### Build/Testing/Packaging Improvement[​](#buildtestingpackaging-improvement-6 "Direct link to Build/Testing/Packaging Improvement")

- Lower binary size in debug build by removing debug info from `Functions`. This is needed only for one internal project in Yandex who is using very old linker. [#14549](https://github.com/ClickHouse/ClickHouse/pull/14549) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Prepare for build with clang 11. [#14455](https://github.com/ClickHouse/ClickHouse/pull/14455) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Fix the logic in backport script. In previous versions it was triggered for any labels of 100% red color. It was strange. [#14433](https://github.com/ClickHouse/ClickHouse/pull/14433) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Integration tests use default base config. All config changes are explicit with main\_configs, user\_configs and dictionaries parameters for instance. [#13647](https://github.com/ClickHouse/ClickHouse/pull/13647) ([Ilya Yatsishin](https://github.com/qoega)).

## ClickHouse release 20.8[​](#clickhouse-release-208 "Direct link to ClickHouse release 20.8")

### ClickHouse release v20.8.12.2-lts, 2021-01-16[​](#clickhouse-release-v208122-lts-2021-01-16 "Direct link to ClickHouse release v20.8.12.2-lts, 2021-01-16")

#### Bug Fix[​](#bug-fix-16 "Direct link to Bug Fix")

- Fix \*If combinator with unary function and Nullable types. [#18806](https://github.com/ClickHouse/ClickHouse/pull/18806) ([Azat Khuzhin](https://github.com/azat)).
- Restrict merges from wide to compact parts. In case of vertical merge it led to broken result part. [#18381](https://github.com/ClickHouse/ClickHouse/pull/18381) ([Anton Popov](https://github.com/CurtizJ)).

### ClickHouse release v20.8.11.17-lts, 2020-12-25[​](#clickhouse-release-v2081117-lts-2020-12-25 "Direct link to ClickHouse release v20.8.11.17-lts, 2020-12-25")

#### Bug Fix[​](#bug-fix-17 "Direct link to Bug Fix")

- Disable write with AIO during merges because it can lead to extremely rare data corruption of primary key columns during merge. [#18481](https://github.com/ClickHouse/ClickHouse/pull/18481) ([alesapin](https://github.com/alesapin)).
- Fixed `value is too short` error when executing `toType(...)` functions (`toDate`, `toUInt32`, etc) with argument of type `Nullable(String)`. Now such functions return `NULL` on parsing errors instead of throwing exception. Fixes [#7673](https://github.com/ClickHouse/ClickHouse/issues/7673). [#18445](https://github.com/ClickHouse/ClickHouse/pull/18445) ([tavplubix](https://github.com/tavplubix)).
- Fix possible crashes in aggregate functions with combinator `Distinct`, while using two-level aggregation. Fixes [#17682](https://github.com/ClickHouse/ClickHouse/issues/17682). [#18365](https://github.com/ClickHouse/ClickHouse/pull/18365) ([Anton Popov](https://github.com/CurtizJ)).

### ClickHouse release v20.8.10.13-lts, 2020-12-24[​](#clickhouse-release-v2081013-lts-2020-12-24 "Direct link to ClickHouse release v20.8.10.13-lts, 2020-12-24")

#### Bug Fix[​](#bug-fix-18 "Direct link to Bug Fix")