2021 Changelog

#### Build/Testing/Packaging Improvement[​](#buildtestingpackaging-improvement-5 "Direct link to 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[​](#clickhouse-release-215-2021-05-20 "Direct link to ClickHouse release 21.5, 2021-05-20")

#### Backward Incompatible Change[​](#backward-incompatible-change-6 "Direct link to Backward Incompatible Change")

- Change comparison of integers and floating point numbers when integer is not exactly representable in the floating point data type. In new version comparison will return false as the rounding error will occur. Example: `9223372036854775808.0 != 9223372036854775808`, because the number `9223372036854775808` is not representable as floating point number exactly (and `9223372036854775808.0` is rounded to `9223372036854776000.0`). But in previous version the comparison will return as the numbers are equal, because if the floating point number `9223372036854776000.0` get converted back to UInt64, it will yield `9223372036854775808`. For the reference, the Python programming language also treats these numbers as equal. But this behaviour was dependend on CPU model (different results on AMD64 and AArch64 for some out-of-range numbers), so we make the comparison more precise. It will treat int and float numbers equal only if int is represented in floating point type exactly. [#22595](https://github.com/ClickHouse/ClickHouse/pull/22595) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Remove support for `argMin` and `argMax` for single `Tuple` argument. The code was not memory-safe. The feature was added by mistake and it is confusing for people. These functions can be reintroduced under different names later. This fixes [#22384](https://github.com/ClickHouse/ClickHouse/issues/22384) and reverts [#17359](https://github.com/ClickHouse/ClickHouse/issues/17359). [#23393](https://github.com/ClickHouse/ClickHouse/pull/23393) ([alexey-milovidov](https://github.com/alexey-milovidov)).

#### New Feature[​](#new-feature-6 "Direct link to New Feature")

- Added functions `dictGetChildren(dictionary, key)`, `dictGetDescendants(dictionary, key, level)`. Function `dictGetChildren` return all children as an array if indexes. It is a inverse transformation for `dictGetHierarchy`. Function `dictGetDescendants` return all descendants as if `dictGetChildren` was applied `level` times recursively. Zero `level` value is equivalent to infinity. Improved performance of `dictGetHierarchy`, `dictIsIn` functions. Closes [#14656](https://github.com/ClickHouse/ClickHouse/issues/14656). [#22096](https://github.com/ClickHouse/ClickHouse/pull/22096) ([Maksim Kita](https://github.com/kitaisreal)).
- Added function `dictGetOrNull`. It works like `dictGet`, but return `Null` in case key was not found in dictionary. Closes [#22375](https://github.com/ClickHouse/ClickHouse/issues/22375). [#22413](https://github.com/ClickHouse/ClickHouse/pull/22413) ([Maksim Kita](https://github.com/kitaisreal)).
- Added a table function `s3Cluster`, which allows to process files from `s3` in parallel on every node of a specified cluster. [#22012](https://github.com/ClickHouse/ClickHouse/pull/22012) ([Nikita Mikhaylov](https://github.com/nikitamikhaylov)).
- Added support for replicas and shards in MySQL/PostgreSQL table engine / table function. You can write `SELECT * FROM mysql('host{1,2}-{1|2}', ...)`. Closes [#20969](https://github.com/ClickHouse/ClickHouse/issues/20969). [#22217](https://github.com/ClickHouse/ClickHouse/pull/22217) ([Kseniia Sumarokova](https://github.com/kssenii)).
- Added `ALTER TABLE ... FETCH PART ...` query. It's similar to `FETCH PARTITION`, but fetches only one part. [#22706](https://github.com/ClickHouse/ClickHouse/pull/22706) ([turbo jason](https://github.com/songenjie)).
- Added a setting `max_distributed_depth` that limits the depth of recursive queries to `Distributed` tables. Closes [#20229](https://github.com/ClickHouse/ClickHouse/issues/20229). [#21942](https://github.com/ClickHouse/ClickHouse/pull/21942) ([flynn](https://github.com/ucasFL)).

#### Performance Improvement[​](#performance-improvement-6 "Direct link to Performance Improvement")