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### ClickHouse release 21.6, 2021-06-05
#### Upgrade Notes
* `zstd` compression library is updated to v1.5.0. You may get messages about "checksum does not match" in replication. These messages are expected due to update of compression algorithm and you can ignore them. These messages are informational and do not indicate any kinds of undesired behaviour.
* The setting `compile_expressions` is enabled by default. Although it has been heavily tested on variety of scenarios, if you find some undesired behaviour on your servers, you can try turning this setting off.
* Values of `UUID` type cannot be compared with integer. For example, instead of writing `uuid != 0` type `uuid != '00000000-0000-0000-0000-000000000000'`.
#### New Feature
* Add Postgres-like cast operator (`::`). E.g.: `[1, 2]::Array(UInt8)`, `0.1::Decimal(4, 4)`, `number::UInt16`. [#23871](https://github.com/ClickHouse/ClickHouse/pull/23871) ([Anton Popov](https://github.com/CurtizJ)).
* Make big integers production ready. Add support for `UInt128` data type. Fix known issues with the `Decimal256` data type. Support big integers in dictionaries. Support `gcd`/`lcm` functions for big integers. Support big integers in array search and conditional functions. Support `LowCardinality(UUID)`. Support big integers in `generateRandom` table function and `clickhouse-obfuscator`. Fix error with returning `UUID` from scalar subqueries. This fixes [#7834](https://github.com/ClickHouse/ClickHouse/issues/7834). This fixes [#23936](https://github.com/ClickHouse/ClickHouse/issues/23936). This fixes [#4176](https://github.com/ClickHouse/ClickHouse/issues/4176). This fixes [#24018](https://github.com/ClickHouse/ClickHouse/issues/24018). Backward incompatible change: values of `UUID` type cannot be compared with integer. For example, instead of writing `uuid != 0` type `uuid != '00000000-0000-0000-0000-000000000000'`. [#23631](https://github.com/ClickHouse/ClickHouse/pull/23631) ([alexey-milovidov](https://github.com/alexey-milovidov)).
* Support `Array` data type for inserting and selecting data in `Arrow`, `Parquet` and `ORC` formats. [#21770](https://github.com/ClickHouse/ClickHouse/pull/21770) ([taylor12805](https://github.com/taylor12805)).
* Implement table comments. Closes [#23225](https://github.com/ClickHouse/ClickHouse/issues/23225). [#23548](https://github.com/ClickHouse/ClickHouse/pull/23548) ([flynn](https://github.com/ucasFL)).
* Support creating dictionaries with DDL queries in `clickhouse-local`. Closes [#22354](https://github.com/ClickHouse/ClickHouse/issues/22354). Added support for `DETACH DICTIONARY PERMANENTLY`. Added support for `EXCHANGE DICTIONARIES` for `Atomic` database engine. Added support for moving dictionaries between databases using `RENAME DICTIONARY`. [#23436](https://github.com/ClickHouse/ClickHouse/pull/23436) ([Maksim Kita](https://github.com/kitaisreal)).
* Add aggregate function `uniqTheta` to support [Theta Sketch](https://datasketches.apache.org/docs/Theta/ThetaSketchFramework.html) in ClickHouse. [#23894](https://github.com/ClickHouse/ClickHouse/pull/23894). [#22609](https://github.com/ClickHouse/ClickHouse/pull/22609) ([Ping Yu](https://github.com/pingyu)).
* Add function `splitByRegexp`. [#24077](https://github.com/ClickHouse/ClickHouse/pull/24077) ([abel-cheng](https://github.com/abel-cheng)).
* Add function `arrayProduct` which accept an array as the parameter, and return the product of all the elements in array. Closes [#21613](https://github.com/ClickHouse/ClickHouse/issues/21613). [#23782](https://github.com/ClickHouse/ClickHouse/pull/23782) ([Maksim Kita](https://github.com/kitaisreal)).
* Add `thread_name` column in `system.stack_trace`. This closes [#23256](https://github.com/ClickHouse/ClickHouse/issues/23256). [#24124](https://github.com/ClickHouse/ClickHouse/pull/24124) ([abel-cheng](https://github.com/abel-cheng)).
* If `insert_null_as_default` = 1, insert default values instead of NULL in `INSERT ... SELECT` and `INSERT ... SELECT ... UNION ALL ...` queries. Closes [#22832](https://github.com/ClickHouse/ClickHouse/issues/22832). [#23524](https://github.com/ClickHouse/ClickHouse/pull/23524) ([Kseniia Sumarokova](https://github.com/kssenii)).
* Add support for progress indication in `clickhouse-local` with `--progress` option. [#23196](https://github.com/ClickHouse/ClickHouse/pull/23196) ([Egor Savin](https://github.com/Amesaru)).
* Add support for HTTP compression (determined by `Content-Encoding` HTTP header) in `http` dictionary source. This fixes [#8912](https://github.com/ClickHouse/ClickHouse/issues/8912). [#23946](https://github.com/ClickHouse/ClickHouse/pull/23946) ([FArthur-cmd](https://github.com/FArthur-cmd)).
* Added `SYSTEM QUERY RELOAD MODEL`, `SYSTEM QUERY RELOAD MODELS`. Closes [#18722](https://github.com/ClickHouse/ClickHouse/issues/18722). [#23182](https://github.com/ClickHouse/ClickHouse/pull/23182) ([Maksim Kita](https://github.com/kitaisreal)).
* Add setting `json` (boolean, 0 by default) for `EXPLAIN PLAN` query. When enabled, query output will be a single `JSON` row. It is recommended to use `TSVRaw` format to avoid unnecessary escaping. [#23082](https://github.com/ClickHouse/ClickHouse/pull/23082) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).
* Add setting `indexes` (boolean, disabled by default) to `EXPLAIN PIPELINE` query. When enabled, shows used indexes, number of filtered parts and granules for every index applied. Supported for `MergeTree*` tables. [#22352](https://github.com/ClickHouse/ClickHouse/pull/22352) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).
* LDAP: implemented user DN detection functionality to use when mapping Active Directory groups to ClickHouse roles. [#22228](https://github.com/ClickHouse/ClickHouse/pull/22228) ([Denis Glazachev](https://github.com/traceon)).
* New aggregate function `deltaSumTimestamp` for summing the difference between consecutive rows while maintaining ordering during merge by storing timestamps. [#21888](https://github.com/ClickHouse/ClickHouse/pull/21888) ([Russ Frank](https://github.com/rf)).
* Added less secure IMDS credentials provider for S3 which works under docker correctly. [#21852](https://github.com/ClickHouse/ClickHouse/pull/21852) ([Vladimir Chebotarev](https://github.com/excitoon)).
* Add back `indexHint` function. This is for [#21238](https://github.com/ClickHouse/ClickHouse/issues/21238). This reverts [#9542](https://github.com/ClickHouse/ClickHouse/pull/9542). This fixes [#9540](https://github.com/ClickHouse/ClickHouse/issues/9540). [#21304](https://github.com/ClickHouse/ClickHouse/pull/21304) ([Amos Bird](https://github.com/amosbird)).
#### Experimental Feature
* Add `PROJECTION` support for `MergeTree*` tables. [#20202](https://github.com/ClickHouse/ClickHouse/pull/20202) ([Amos Bird](https://github.com/amosbird)).
#### Performance Improvement
* Enable `compile_expressions` setting by default. When this setting enabled, compositions of simple functions and operators will be compiled to native code with LLVM at runtime. [#8482](https://github.com/ClickHouse/ClickHouse/pull/8482) ([Maksim Kita](https://github.com/kitaisreal), [alexey-milovidov](https://github.com/alexey-milovidov)). Note: if you feel in trouble, turn this option off.
* Update `re2` library. Performance of regular expressions matching is improved. Also this PR adds compatibility with gcc-11. [#24196](https://github.com/ClickHouse/ClickHouse/pull/24196) ([Raúl Marín](https://github.com/Algunenano)).
* ORC input format reading by stripe instead of reading entire table into memory by once which is cost memory when file size is huge. [#23102](https://github.com/ClickHouse/ClickHouse/pull/23102) ([Chao Ma](https://github.com/godliness)).
* Fusion of aggregate functions `sum`, `count` and `avg` in a query into single aggregate function. The optimization is controlled with the `optimize_fuse_sum_count_avg` setting. This is implemented with a new aggregate function `sumCount`. This function returns a tuple of two fields: `sum` and `count`. [#21337](https://github.com/ClickHouse/ClickHouse/pull/21337) ([hexiaoting](https://github.com/hexiaoting)).
* Update `zstd` to v1.5.0. The performance of compression is improved for single digits percentage. [#24135](https://github.com/ClickHouse/ClickHouse/pull/24135) ([Raúl Marín](https://github.com/Algunenano)). Note: you may get messages about "checksum does not match" in replication. These messages are expected due to update of compression algorithm and you can ignore them.
* Improved performance of `Buffer` tables: do not acquire lock for total_bytes/total_rows for `Buffer` engine. [#24066](https://github.com/ClickHouse/ClickHouse/pull/24066) ([Azat Khuzhin](https://github.com/azat)).
* Preallocate support for hashed/sparse_hashed dictionaries is returned. [#23979](https://github.com/ClickHouse/ClickHouse/pull/23979) ([Azat Khuzhin](https://github.com/azat)).
* Enable `async_socket_for_remote` by default (lower amount of threads in querying Distributed tables with large fanout). [#23683](https://github.com/ClickHouse/ClickHouse/pull/23683) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).
#### Improvement
* Add `_partition_value` virtual column to MergeTree table family. It can be used to prune partition in a deterministic way. It's needed to implement partition matcher for mutations. [#23673](https://github.com/ClickHouse/ClickHouse/pull/23673) ([Amos Bird](https://github.com/amosbird)).
* Added `region` parameter for S3 storage and disk. [#23846](https://github.com/ClickHouse/ClickHouse/pull/23846) ([Vladimir Chebotarev](https://github.com/excitoon)).
* Allow configuring different log levels for different logging channels. Closes [#19569](https://github.com/ClickHouse/ClickHouse/issues/19569). [#23857](https://github.com/ClickHouse/ClickHouse/pull/23857) ([filimonov](https://github.com/filimonov)).
* Keep default timezone on `DateTime` operations if it was not provided explicitly. For example, if you add one second to a value of `DateTime` type without timezone it will remain `DateTime` without timezone. In previous versions the value of default timezone was placed to the returned data type explicitly so it becomes DateTime('something'). This closes [#4854](https://github.com/ClickHouse/ClickHouse/issues/4854). [#23392](https://github.com/ClickHouse/ClickHouse/pull/23392) ([alexey-milovidov](https://github.com/alexey-milovidov)).
* Allow user to specify empty string instead of database name for `MySQL` storage. Default database will be used for queries. In previous versions it was working for SELECT queries and not support for INSERT was also added. This closes [#19281](https://github.com/ClickHouse/ClickHouse/issues/19281). This can be useful working with `Sphinx` or other MySQL-compatible foreign databases. [#23319](https://github.com/ClickHouse/ClickHouse/pull/23319) ([alexey-milovidov](https://github.com/alexey-milovidov)).
* Fixed `quantile(s)TDigest`. Added special handling of singleton centroids according to tdunning/t-digest 3.2+. Also a bug with over-compression of centroids in implementation of earlier version of the algorithm was fixed. [#23314](https://github.com/ClickHouse/ClickHouse/pull/23314) ([Vladimir Chebotarev](https://github.com/excitoon)).
* Function `now64` now supports optional timezone argument. [#24091](https://github.com/ClickHouse/ClickHouse/pull/24091) ([Vasily Nemkov](https://github.com/Enmk)).
* Fix the case when a progress bar in interactive mode in clickhouse-client that appear in the middle of the data may rewrite some parts of visible data in terminal. This closes [#19283](https://github.com/ClickHouse/ClickHouse/issues/19283). [#23050](https://github.com/ClickHouse/ClickHouse/pull/23050) ([alexey-milovidov](https://github.com/alexey-milovidov)).
* Fix crash when memory allocation fails in simdjson. https://github.com/simdjson/simdjson/pull/1567 . Mark as improvement because it's a very rare bug. [#24147](https://github.com/ClickHouse/ClickHouse/pull/24147) ([Amos Bird](https://github.com/amosbird)).
* Preserve dictionaries until storage shutdown (this will avoid possible `external dictionary 'DICT' not found` errors at server shutdown during final flush of the `Buffer` engine). [#24068](https://github.com/ClickHouse/ClickHouse/pull/24068) ([Azat Khuzhin](https://github.com/azat)).
* Flush `Buffer` tables before shutting down tables (within one database), to avoid discarding blocks due to underlying table had been already detached (and `Destination table default.a_data_01870 doesn't exist. Block of data is discarded` error in the log). [#24067](https://github.com/ClickHouse/ClickHouse/pull/24067) ([Azat Khuzhin](https://github.com/azat)).
* Now `prefer_column_name_to_alias = 1` will also favor column names for `group by`, `having` and `order by`. This fixes [#23882](https://github.com/ClickHouse/ClickHouse/issues/23882). [#24022](https://github.com/ClickHouse/ClickHouse/pull/24022) ([Amos Bird](https://github.com/amosbird)).
* Add support for `ORDER BY WITH FILL` with `DateTime64`. [#24016](https://github.com/ClickHouse/ClickHouse/pull/24016) ([kevin wan](https://github.com/MaxWk)).
* Enable `DateTime64` to be a version column in `ReplacingMergeTree`. [#23992](https://github.com/ClickHouse/ClickHouse/pull/23992) ([kevin wan](https://github.com/MaxWk)).
* Log information about OS name, kernel version and CPU architecture on server startup. [#23988](https://github.com/ClickHouse/ClickHouse/pull/23988) ([Azat Khuzhin](https://github.com/azat)).
* Support specifying table schema for `postgresql` dictionary source. Closes [#23958](https://github.com/ClickHouse/ClickHouse/issues/23958). [#23980](https://github.com/ClickHouse/ClickHouse/pull/23980) ([Kseniia Sumarokova](https://github.com/kssenii)).
* Add hints for names of `Enum` elements (suggest names in case of typos). Closes [#17112](https://github.com/ClickHouse/ClickHouse/issues/17112). [#23919](https://github.com/ClickHouse/ClickHouse/pull/23919) ([flynn](https://github.com/ucasFL)).
* Measure found rate (the percentage for which the value was found) for dictionaries (see `found_rate` in `system.dictionaries`). [#23916](https://github.com/ClickHouse/ClickHouse/pull/23916) ([Azat Khuzhin](https://github.com/azat)).
* Allow to add specific queue settings via table settng `rabbitmq_queue_settings_list`. (Closes [#23737](https://github.com/ClickHouse/ClickHouse/issues/23737) and [#23918](https://github.com/ClickHouse/ClickHouse/issues/23918)). Allow user to control all RabbitMQ setup: if table setting `rabbitmq_queue_consume` is set to `1` - RabbitMQ table engine will only connect to specified queue and will not perform any RabbitMQ consumer-side setup like declaring exchange, queues, bindings. (Closes [#21757](https://github.com/ClickHouse/ClickHouse/issues/21757)). Add proper cleanup when RabbitMQ table is dropped - delete queues, which the table has declared and all bound exchanges - if they were created by the table. [#23887](https://github.com/ClickHouse/ClickHouse/pull/23887) ([Kseniia Sumarokova](https://github.com/kssenii)).
* Add `broken_data_files`/`broken_data_compressed_bytes` into `system.distribution_queue`. Add metric for number of files for asynchronous insertion into Distributed tables that has been marked as broken (`BrokenDistributedFilesToInsert`). [#23885](https://github.com/ClickHouse/ClickHouse/pull/23885) ([Azat Khuzhin](https://github.com/azat)).
* Querying `system.tables` does not go to ZooKeeper anymore. [#23793](https://github.com/ClickHouse/ClickHouse/pull/23793) ([Fuwang Hu](https://github.com/fuwhu)).
* Respect `lock_acquire_timeout_for_background_operations` for `OPTIMIZE` queries. [#23623](https://github.com/ClickHouse/ClickHouse/pull/23623) ([Azat Khuzhin](https://github.com/azat)).
* Possibility to change `S3` disk settings in runtime via new `SYSTEM RESTART DISK` SQL command. [#23429](https://github.com/ClickHouse/ClickHouse/pull/23429) ([Pavel Kovalenko](https://github.com/Jokser)).
* If user applied a misconfiguration by mistakenly setting `max_distributed_connections` to value zero, every query to a `Distributed` table will throw exception with a message containing "logical error". But it's really an expected behaviour, not a logical error, so the exception message was slightly incorrect. It also triggered checks in our CI enviroment that ensures that no logical errors ever happen. Instead we will treat `max_distributed_connections` misconfigured to zero as the minimum possible value (one). [#23348](https://github.com/ClickHouse/ClickHouse/pull/23348) ([Azat Khuzhin](https://github.com/azat)).
* Disable `min_bytes_to_use_mmap_io` by default. [#23322](https://github.com/ClickHouse/ClickHouse/pull/23322) ([Azat Khuzhin](https://github.com/azat)).
* Support `LowCardinality` nullability with `join_use_nulls`, close [#15101](https://github.com/ClickHouse/ClickHouse/issues/15101). [#23237](https://github.com/ClickHouse/ClickHouse/pull/23237) ([vdimir](https://github.com/vdimir)).
* Added possibility to restore `MergeTree` parts to `detached` directory for `S3` disk. [#23112](https://github.com/ClickHouse/ClickHouse/pull/23112) ([Pavel Kovalenko](https://github.com/Jokser)).
* Retries on HTTP connection drops in S3. [#22988](https://github.com/ClickHouse/ClickHouse/pull/22988) ([Vladimir Chebotarev](https://github.com/excitoon)).
* Add settings `external_storage_max_read_rows` and `external_storage_max_read_rows` for MySQL table engine, dictionary source and MaterializeMySQL minor data fetches. [#22697](https://github.com/ClickHouse/ClickHouse/pull/22697) ([TCeason](https://github.com/TCeason)).
* `MaterializeMySQL` (experimental feature): Previously, MySQL 5.7.9 was not supported due to SQL incompatibility. Now leave MySQL parameter verification to the MaterializeMySQL. [#23413](https://github.com/ClickHouse/ClickHouse/pull/23413) ([TCeason](https://github.com/TCeason)).
* Enable reading of subcolumns for distributed tables. [#24472](https://github.com/ClickHouse/ClickHouse/pull/24472) ([Anton Popov](https://github.com/CurtizJ)).
* Fix usage of tuples in `CREATE .. AS SELECT` queries. [#24464](https://github.com/ClickHouse/ClickHouse/pull/24464) ([Anton Popov](https://github.com/CurtizJ)).
* Support for `Parquet` format in `Kafka` tables. [#23412](https://github.com/ClickHouse/ClickHouse/pull/23412) ([Chao Ma](https://github.com/godliness)).
#### Bug Fix
* Use old modulo function version when used in partition key and primary key. Closes [#23508](https://github.com/ClickHouse/ClickHouse/issues/23508). [#24157](https://github.com/ClickHouse/ClickHouse/pull/24157) ([Kseniia Sumarokova](https://github.com/kssenii)). It was a source of backward incompatibility in previous releases.
* Fixed the behavior when query `SYSTEM RESTART REPLICA` or `SYSTEM SYNC REPLICA` is being processed infinitely. This was detected on server with extremely little amount of RAM. [#24457](https://github.com/ClickHouse/ClickHouse/pull/24457) ([Nikita Mikhaylov](https://github.com/nikitamikhaylov)).
* Fix incorrect monotonicity of `toWeek` function. This fixes [#24422](https://github.com/ClickHouse/ClickHouse/issues/24422) . This bug was introduced in [#5212](https://github.com/ClickHouse/ClickHouse/pull/5212), and was exposed later by smarter partition pruner. [#24446](https://github.com/ClickHouse/ClickHouse/pull/24446) ([Amos Bird](https://github.com/amosbird)).
* Fix drop partition with intersect fake parts. In rare cases there might be parts with mutation version greater than current block number. [#24321](https://github.com/ClickHouse/ClickHouse/pull/24321) ([Amos Bird](https://github.com/amosbird)).
* Fixed a bug in moving Materialized View from Ordinary to Atomic database (`RENAME TABLE` query). Now inner table is moved to new database together with Materialized View. Fixes [#23926](https://github.com/ClickHouse/ClickHouse/issues/23926). [#24309](https://github.com/ClickHouse/ClickHouse/pull/24309) ([tavplubix](https://github.com/tavplubix)).
* Allow empty HTTP headers in client requests. Fixes [#23901](https://github.com/ClickHouse/ClickHouse/issues/23901). [#24285](https://github.com/ClickHouse/ClickHouse/pull/24285) ([Ivan](https://github.com/abyss7)).
* Set `max_threads = 1` to fix mutation fail of `Memory` tables. Closes [#24274](https://github.com/ClickHouse/ClickHouse/issues/24274). [#24275](https://github.com/ClickHouse/ClickHouse/pull/24275) ([flynn](https://github.com/ucasFL)).
* Fix typo in implementation of `Memory` tables, this bug was introduced at [#15127](https://github.com/ClickHouse/ClickHouse/issues/15127). Closes [#24192](https://github.com/ClickHouse/ClickHouse/issues/24192). [#24193](https://github.com/ClickHouse/ClickHouse/pull/24193) ([张中南](https://github.com/plugine)).
* Fix abnormal server termination due to `HDFS` becoming not accessible during query execution. Closes [#24117](https://github.com/ClickHouse/ClickHouse/issues/24117). [#24191](https://github.com/ClickHouse/ClickHouse/pull/24191) ([Kseniia Sumarokova](https://github.com/kssenii)).
* Fix crash on updating of `Nested` column with const condition. [#24183](https://github.com/ClickHouse/ClickHouse/pull/24183) ([hexiaoting](https://github.com/hexiaoting)).
* Fix race condition which could happen in RBAC under a heavy load. This PR fixes [#24090](https://github.com/ClickHouse/ClickHouse/issues/24090), [#24134](https://github.com/ClickHouse/ClickHouse/issues/24134),. [#24176](https://github.com/ClickHouse/ClickHouse/pull/24176) ([Vitaly Baranov](https://github.com/vitlibar)).
* Fix a rare bug that could lead to a partially initialized table that can serve write requests (insert/alter/so on). Now such tables will be in readonly mode. [#24122](https://github.com/ClickHouse/ClickHouse/pull/24122) ([alesapin](https://github.com/alesapin)).
* Fix an issue: `EXPLAIN PIPELINE` with `SELECT xxx FINAL` showed a wrong pipeline. ([hexiaoting](https://github.com/hexiaoting)).
* Fixed using const `DateTime` value vs `DateTime64` column in `WHERE`. [#24100](https://github.com/ClickHouse/ClickHouse/pull/24100) ([Vasily Nemkov](https://github.com/Enmk)).
* Fix crash in merge JOIN, closes [#24010](https://github.com/ClickHouse/ClickHouse/issues/24010). [#24013](https://github.com/ClickHouse/ClickHouse/pull/24013) ([vdimir](https://github.com/vdimir)).
* Some `ALTER PARTITION` queries might cause `Part A intersects previous part B` and `Unexpected merged part C intersecting drop range D` errors in replication queue. It's fixed. Fixes [#23296](https://github.com/ClickHouse/ClickHouse/issues/23296). [#23997](https://github.com/ClickHouse/ClickHouse/pull/23997) ([tavplubix](https://github.com/tavplubix)).
* Fix SIGSEGV for external GROUP BY and overflow row (i.e. queries like `SELECT FROM GROUP BY WITH TOTALS SETTINGS max_bytes_before_external_group_by>0, max_rows_to_group_by>0, group_by_overflow_mode='any', totals_mode='before_having'`). [#23962](https://github.com/ClickHouse/ClickHouse/pull/23962) ([Azat Khuzhin](https://github.com/azat)).
* Fix keys metrics accounting for `CACHE` dictionary with duplicates in the source (leads to `DictCacheKeysRequestedMiss` overflows). [#23929](https://github.com/ClickHouse/ClickHouse/pull/23929) ([Azat Khuzhin](https://github.com/azat)).
* Fix implementation of connection pool of `PostgreSQL` engine. Closes [#23897](https://github.com/ClickHouse/ClickHouse/issues/23897). [#23909](https://github.com/ClickHouse/ClickHouse/pull/23909) ([Kseniia Sumarokova](https://github.com/kssenii)).
* Fix `distributed_group_by_no_merge = 2` with `GROUP BY` and aggregate function wrapped into regular function (had been broken in [#23546](https://github.com/ClickHouse/ClickHouse/issues/23546)). Throw exception in case of someone trying to use `distributed_group_by_no_merge = 2` with window functions. Disable `optimize_distributed_group_by_sharding_key` for queries with window functions. [#23906](https://github.com/ClickHouse/ClickHouse/pull/23906) ([Azat Khuzhin](https://github.com/azat)).
* A fix for `s3` table function: better handling of HTTP errors. Response bodies of HTTP errors were being ignored earlier. [#23844](https://github.com/ClickHouse/ClickHouse/pull/23844) ([Vladimir Chebotarev](https://github.com/excitoon)).
* A fix for `s3` table function: better handling of URI's. Fixed an incompatibility with URLs containing `+` symbol, data with such keys could not be read previously. [#23822](https://github.com/ClickHouse/ClickHouse/pull/23822) ([Vladimir Chebotarev](https://github.com/excitoon)).
* Fix error `Can't initialize pipeline with empty pipe` for queries with `GLOBAL IN/JOIN` and `use_hedged_requests`. Fixes [#23431](https://github.com/ClickHouse/ClickHouse/issues/23431). [#23805](https://github.com/ClickHouse/ClickHouse/pull/23805) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).
* Fix `CLEAR COLUMN` does not work when it is referenced by materialized view. Close [#23764](https://github.com/ClickHouse/ClickHouse/issues/23764). [#23781](https://github.com/ClickHouse/ClickHouse/pull/23781) ([flynn](https://github.com/ucasFL)).
* Fix heap use after free when reading from HDFS if `Values` format is used. [#23761](https://github.com/ClickHouse/ClickHouse/pull/23761) ([Kseniia Sumarokova](https://github.com/kssenii)).
* Avoid possible "Cannot schedule a task" error (in case some exception had been occurred) on INSERT into Distributed. [#23744](https://github.com/ClickHouse/ClickHouse/pull/23744) ([Azat Khuzhin](https://github.com/azat)).
* Fixed a bug in recovery of staled `ReplicatedMergeTree` replica. Some metadata updates could be ignored by staled replica if `ALTER` query was executed during downtime of the replica. [#23742](https://github.com/ClickHouse/ClickHouse/pull/23742) ([tavplubix](https://github.com/tavplubix)).
* Fix a bug with `Join` and `WITH TOTALS`, close [#17718](https://github.com/ClickHouse/ClickHouse/issues/17718). [#23549](https://github.com/ClickHouse/ClickHouse/pull/23549) ([vdimir](https://github.com/vdimir)).
* Fix possible `Block structure mismatch` error for queries with `UNION` which could possibly happen after filter-pushdown optimization. Fixes [#23029](https://github.com/ClickHouse/ClickHouse/issues/23029). [#23359](https://github.com/ClickHouse/ClickHouse/pull/23359) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).
* Add type conversion when the setting `optimize_skip_unused_shards_rewrite_in` is enabled. This fixes MSan report. [#23219](https://github.com/ClickHouse/ClickHouse/pull/23219) ([Azat Khuzhin](https://github.com/azat)).
* Add a missing check when updating nested subcolumns, close issue: [#22353](https://github.com/ClickHouse/ClickHouse/issues/22353). [#22503](https://github.com/ClickHouse/ClickHouse/pull/22503) ([hexiaoting](https://github.com/hexiaoting)).
#### Build/Testing/Packaging Improvement
* Support building on Illumos. [#24144](https://github.com/ClickHouse/ClickHouse/pull/24144). Adds support for building on Solaris-derived operating systems. [#23746](https://github.com/ClickHouse/ClickHouse/pull/23746) ([bnaecker](https://github.com/bnaecker)).
* Add more benchmarks for hash tables, including the Swiss Table from Google (that appeared to be slower than ClickHouse hash map in our specific usage scenario). [#24111](https://github.com/ClickHouse/ClickHouse/pull/24111) ([Maksim Kita](https://github.com/kitaisreal)).
* Update librdkafka 1.6.0-RC3 to 1.6.1. [#23874](https://github.com/ClickHouse/ClickHouse/pull/23874) ([filimonov](https://github.com/filimonov)).
* Always enable `asynchronous-unwind-tables` explicitly. It may fix query profiler on AArch64. [#23602](https://github.com/ClickHouse/ClickHouse/pull/23602) ([alexey-milovidov](https://github.com/alexey-milovidov)).
* Avoid possible build dependency on locale and filesystem order. This allows reproducible builds. [#23600](https://github.com/ClickHouse/ClickHouse/pull/23600) ([alexey-milovidov](https://github.com/alexey-milovidov)).
* Remove a source of nondeterminism from build. Now builds at different point of time will produce byte-identical binaries. Partially addressed [#22113](https://github.com/ClickHouse/ClickHouse/issues/22113). [#23559](https://github.com/ClickHouse/ClickHouse/pull/23559) ([alexey-milovidov](https://github.com/alexey-milovidov)).
* Add simple tool for benchmarking (Zoo)Keeper. [#23038](https://github.com/ClickHouse/ClickHouse/pull/23038) ([alesapin](https://github.com/alesapin)).
## ClickHouse release 21.5, 2021-05-20
#### Backward Incompatible Change