2020 Changelog

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

- Index not used for IN operator with literals, performance regression introduced around v19.3. This fixes [#10574](https://github.com/ClickHouse/ClickHouse/issues/10574). [#12062](https://github.com/ClickHouse/ClickHouse/pull/12062) ([nvartolomei](https://github.com/nvartolomei)).

#### Build/Testing/Packaging Improvement[​](#buildtestingpackaging-improvement-10 "Direct link to Build/Testing/Packaging Improvement")

- Install `ca-certificates` before the first `apt-get update` in Dockerfile. [#12095](https://github.com/ClickHouse/ClickHouse/pull/12095) ([Ivan Blinkov](https://github.com/blinkov)).

### ClickHouse release v20.5.2.7-stable 2020-07-02[​](#clickhouse-release-v20527-stable-2020-07-02 "Direct link to ClickHouse release v20.5.2.7-stable 2020-07-02")

#### Backward Incompatible Change[​](#backward-incompatible-change-7 "Direct link to Backward Incompatible Change")

- Return non-Nullable result from COUNT(DISTINCT), and `uniq` aggregate functions family. If all passed values are NULL, return zero instead. This improves SQL compatibility. [#11661](https://github.com/ClickHouse/ClickHouse/pull/11661) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Added a check for the case when user-level setting is specified in a wrong place. User-level settings should be specified in `users.xml` inside `<profile>` section for specific user profile (or in `<default>` for default settings). The server won't start with exception message in log. This fixes [#9051](https://github.com/ClickHouse/ClickHouse/issues/9051). If you want to skip the check, you can either move settings to the appropriate place or add `<skip_check_for_incorrect_settings>1</skip_check_for_incorrect_settings>` to config.xml. [#11449](https://github.com/ClickHouse/ClickHouse/pull/11449) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- The setting `input_format_with_names_use_header` is enabled by default. It will affect parsing of input formats `-WithNames` and `-WithNamesAndTypes`. [#10937](https://github.com/ClickHouse/ClickHouse/pull/10937) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Remove `experimental_use_processors` setting. It is enabled by default. [#10924](https://github.com/ClickHouse/ClickHouse/pull/10924) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).
- Update `zstd` to 1.4.4. It has some minor improvements in performance and compression ratio. If you run replicas with different versions of ClickHouse you may see reasonable error messages `Data after merge is not byte-identical to data on another replicas.` with explanation. These messages are Ok and you should not worry. This change is backward compatible but we list it here in changelog in case you will wonder about these messages. [#10663](https://github.com/ClickHouse/ClickHouse/pull/10663) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Added a check for meaningless codecs and a setting `allow_suspicious_codecs` to control this check. This closes [#4966](https://github.com/ClickHouse/ClickHouse/issues/4966). [#10645](https://github.com/ClickHouse/ClickHouse/pull/10645) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Several Kafka setting changes their defaults. See [#11388](https://github.com/ClickHouse/ClickHouse/pull/11388).
- When upgrading from versions older than 20.5, if rolling update is performed and cluster contains both versions 20.5 or greater and less than 20.5, if ClickHouse nodes with old versions are restarted and old version has been started up in presence of newer versions, it may lead to `Part ... intersects previous part` errors. To prevent this error, first install newer clickhouse-server packages on all cluster nodes and then do restarts (so, when clickhouse-server is restarted, it will start up with the new version).

#### New Feature[​](#new-feature-7 "Direct link to New Feature")