2020 Changelog

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

- Enable `Atomic` database engine by default for newly created databases. [#15003](https://github.com/ClickHouse/ClickHouse/pull/15003) ([tavplubix](https://github.com/tavplubix)).
- Add the ability to specify specialized codecs like `Delta`, `T64`, etc. for columns with subtypes. Implements [#12551](https://github.com/ClickHouse/ClickHouse/issues/12551), fixes [#11397](https://github.com/ClickHouse/ClickHouse/issues/11397), fixes [#4609](https://github.com/ClickHouse/ClickHouse/issues/4609). [#15089](https://github.com/ClickHouse/ClickHouse/pull/15089) ([alesapin](https://github.com/alesapin)).
- Dynamic reload of zookeeper config. [#14678](https://github.com/ClickHouse/ClickHouse/pull/14678) ([sundyli](https://github.com/sundy-li)).
- Now it's allowed to execute `ALTER ... ON CLUSTER` queries regardless of the `<internal_replication>` setting in cluster config. [#16075](https://github.com/ClickHouse/ClickHouse/pull/16075) ([alesapin](https://github.com/alesapin)).
- Now `joinGet` supports multi-key lookup. Continuation of [#12418](https://github.com/ClickHouse/ClickHouse/issues/12418). [#13015](https://github.com/ClickHouse/ClickHouse/pull/13015) ([Amos Bird](https://github.com/amosbird)).
- Wait for `DROP/DETACH TABLE` to actually finish if `NO DELAY` or `SYNC` is specified for `Atomic` database. [#15448](https://github.com/ClickHouse/ClickHouse/pull/15448) ([tavplubix](https://github.com/tavplubix)).
- Now it's possible to change the type of version column for `VersionedCollapsingMergeTree` with `ALTER` query. [#15442](https://github.com/ClickHouse/ClickHouse/pull/15442) ([alesapin](https://github.com/alesapin)).
- Unfold `{database}`, `{table}` and `{uuid}` macros in `zookeeper_path` on replicated table creation. Do not allow `RENAME TABLE` if it may break `zookeeper_path` after server restart. Fixes [#6917](https://github.com/ClickHouse/ClickHouse/issues/6917). [#15348](https://github.com/ClickHouse/ClickHouse/pull/15348) ([tavplubix](https://github.com/tavplubix)).
- The function `now` allows an argument with timezone. This closes [15264](https://github.com/ClickHouse/ClickHouse/issues/15264). [#15285](https://github.com/ClickHouse/ClickHouse/pull/15285) ([flynn](https://github.com/ucasFL)).
- Do not allow connections to ClickHouse server until all scripts in `/docker-entrypoint-initdb.d/` are executed. [#15244](https://github.com/ClickHouse/ClickHouse/pull/15244) ([Aleksei Kozharin](https://github.com/alekseik1)).
- Added `optimize` setting to `EXPLAIN PLAN` query. If enabled, query plan level optimisations are applied. Enabled by default. [#15201](https://github.com/ClickHouse/ClickHouse/pull/15201) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).
- Proper exception message for wrong number of arguments of CAST. This closes [#13992](https://github.com/ClickHouse/ClickHouse/issues/13992). [#15029](https://github.com/ClickHouse/ClickHouse/pull/15029) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Add option to disable TTL move on data part insert. [#15000](https://github.com/ClickHouse/ClickHouse/pull/15000) ([Pavel Kovalenko](https://github.com/Jokser)).
- Ignore key constraints when doing mutations. Without this pull request, it's not possible to do mutations when `force_index_by_date = 1` or `force_primary_key = 1`. [#14973](https://github.com/ClickHouse/ClickHouse/pull/14973) ([Amos Bird](https://github.com/amosbird)).
- Allow to drop Replicated table if previous drop attempt was failed due to ZooKeeper session expiration. This fixes [#11891](https://github.com/ClickHouse/ClickHouse/issues/11891). [#14926](https://github.com/ClickHouse/ClickHouse/pull/14926) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Fixed excessive settings constraint violation when running SELECT with SETTINGS from a distributed table. [#14876](https://github.com/ClickHouse/ClickHouse/pull/14876) ([Amos Bird](https://github.com/amosbird)).
- Provide a `load_balancing_first_offset` query setting to explicitly state what the first replica is. It's used together with `FIRST_OR_RANDOM` load balancing strategy, which allows to control replicas workload. [#14867](https://github.com/ClickHouse/ClickHouse/pull/14867) ([Amos Bird](https://github.com/amosbird)).
- Show subqueries for `SET` and `JOIN` in `EXPLAIN` result. [#14856](https://github.com/ClickHouse/ClickHouse/pull/14856) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).
- Allow using multi-volume storage configuration in storage `Distributed`. [#14839](https://github.com/ClickHouse/ClickHouse/pull/14839) ([Pavel Kovalenko](https://github.com/Jokser)).
- Construct `query_start_time` and `query_start_time_microseconds` from the same timespec. [#14831](https://github.com/ClickHouse/ClickHouse/pull/14831) ([Bharat Nallan](https://github.com/bharatnc)).
- Support for disabling persistency for `StorageJoin` and `StorageSet`, this feature is controlled by setting `disable_set_and_join_persistency`. And this PR solved issue [#6318](https://github.com/ClickHouse/ClickHouse/issues/6318). [#14776](https://github.com/ClickHouse/ClickHouse/pull/14776) ([vxider](https://github.com/Vxider)).
- Now `COLUMNS` can be used to wrap over a list of columns and apply column transformers afterwards. [#14775](https://github.com/ClickHouse/ClickHouse/pull/14775) ([Amos Bird](https://github.com/amosbird)).
- Add `merge_algorithm` to `system.merges` table to improve merging inspections. [#14705](https://github.com/ClickHouse/ClickHouse/pull/14705) ([Amos Bird](https://github.com/amosbird)).
- Fix potential memory leak caused by zookeeper exists watch. [#14693](https://github.com/ClickHouse/ClickHouse/pull/14693) ([hustnn](https://github.com/hustnn)).
- Allow parallel execution of distributed DDL. [#14684](https://github.com/ClickHouse/ClickHouse/pull/14684) ([Azat Khuzhin](https://github.com/azat)).
- Add `QueryMemoryLimitExceeded` event counter. This closes [#14589](https://github.com/ClickHouse/ClickHouse/issues/14589). [#14647](https://github.com/ClickHouse/ClickHouse/pull/14647) ([fastio](https://github.com/fastio)).
- Fix some trailing whitespaces in query formatting. [#14595](https://github.com/ClickHouse/ClickHouse/pull/14595) ([Azat Khuzhin](https://github.com/azat)).
- ClickHouse treats partition expr and key expr differently. Partition expr is used to construct an minmax index containing related columns, while primary key expr is stored as an expr. Sometimes user might partition a table at coarser levels, such as `partition by i / 1000`. However, binary operators are not monotonic and this PR tries to fix that. It might also benifit other use cases. [#14513](https://github.com/ClickHouse/ClickHouse/pull/14513) ([Amos Bird](https://github.com/amosbird)).
- Add an option to skip access checks for `DiskS3`. `s3` disk is an experimental feature. [#14497](https://github.com/ClickHouse/ClickHouse/pull/14497) ([Pavel Kovalenko](https://github.com/Jokser)).
- Speed up server shutdown process if there are ongoing S3 requests. [#14496](https://github.com/ClickHouse/ClickHouse/pull/14496) ([Pavel Kovalenko](https://github.com/Jokser)).
- `SYSTEM RELOAD CONFIG` now throws an exception if failed to reload and continues using the previous users.xml. The background periodic reloading also continues using the previous users.xml if failed to reload. [#14492](https://github.com/ClickHouse/ClickHouse/pull/14492) ([Vitaly Baranov](https://github.com/vitlibar)).
- For INSERTs with inline data in VALUES format in the script mode of `clickhouse-client`, support semicolon as the data terminator, in addition to the new line. Closes [#12288](https://github.com/ClickHouse/ClickHouse/issues/12288). [#13192](https://github.com/ClickHouse/ClickHouse/pull/13192) ([Alexander Kuzmenkov](https://github.com/akuzm)).
- Support custom codecs in compact parts. [#12183](https://github.com/ClickHouse/ClickHouse/pull/12183) ([Anton Popov](https://github.com/CurtizJ)).

#### Performance Improvement[​](#performance-improvement-2 "Direct link to Performance Improvement")