2021 Changelog

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

- `WINDOW VIEW` to enable stream processing in ClickHouse. [#8331](https://github.com/ClickHouse/ClickHouse/pull/8331) ([vxider](https://github.com/Vxider)).
- Drop support for using Ordinary databases with `MaterializedMySQL`. [#31292](https://github.com/ClickHouse/ClickHouse/pull/31292) ([Stig Bakken](https://github.com/stigsb)).
- Implement the commands BACKUP and RESTORE for the Log family. This feature is under development. [#30688](https://github.com/ClickHouse/ClickHouse/pull/30688) ([Vitaly Baranov](https://github.com/vitlibar)).

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

- Reduce memory usage when reading with `s3` / `url` / `hdfs` formats `Parquet`, `ORC`, `Arrow` (controlled by setting `input_format_allow_seeks`, enabled by default). Also add setting `remote_read_min_bytes_for_seek` to control seeks. Closes [#10461](https://github.com/ClickHouse/ClickHouse/issues/10461). Closes [#16857](https://github.com/ClickHouse/ClickHouse/issues/16857). [#30936](https://github.com/ClickHouse/ClickHouse/pull/30936) ([Kseniia Sumarokova](https://github.com/kssenii)).
- Add optimizations for constant conditions in JOIN ON, ref [#26928](https://github.com/ClickHouse/ClickHouse/issues/26928). [#27021](https://github.com/ClickHouse/ClickHouse/pull/27021) ([Vladimir C](https://github.com/vdimir)).
- Support parallel formatting for all text formats, except `JSONEachRowWithProgress` and `PrettyCompactMonoBlock`. [#31489](https://github.com/ClickHouse/ClickHouse/pull/31489) ([Kruglov Pavel](https://github.com/Avogar)).
- Speed up count over nullable columns. [#31806](https://github.com/ClickHouse/ClickHouse/pull/31806) ([Raúl Marín](https://github.com/Algunenano)).
- Speed up `avg` and `sumCount` aggregate functions. [#31694](https://github.com/ClickHouse/ClickHouse/pull/31694) ([Raúl Marín](https://github.com/Algunenano)).
- Improve performance of JSON and XML output formats. [#31673](https://github.com/ClickHouse/ClickHouse/pull/31673) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Improve performance of syncing data to block device. This closes [#31181](https://github.com/ClickHouse/ClickHouse/issues/31181). [#31229](https://github.com/ClickHouse/ClickHouse/pull/31229) ([zhanglistar](https://github.com/zhanglistar)).
- Fixing query performance issue in `LiveView` tables. Fixes [#30831](https://github.com/ClickHouse/ClickHouse/issues/30831). [#31006](https://github.com/ClickHouse/ClickHouse/pull/31006) ([vzakaznikov](https://github.com/vzakaznikov)).
- Speed up query parsing. [#31949](https://github.com/ClickHouse/ClickHouse/pull/31949) ([Raúl Marín](https://github.com/Algunenano)).
- Allow to split `GraphiteMergeTree` rollup rules for plain/tagged metrics (optional `rule_type` field). [#25122](https://github.com/ClickHouse/ClickHouse/pull/25122) ([Michail Safronov](https://github.com/msaf1980)).
- Remove excessive `DESC TABLE` requests for `remote()` (in case of `remote('127.1', system.one)` (i.e. identifier as the db.table instead of string) there was excessive `DESC TABLE` request). [#32019](https://github.com/ClickHouse/ClickHouse/pull/32019) ([Azat Khuzhin](https://github.com/azat)).
- Optimize function `tupleElement` to reading of subcolumn with enabled setting `optimize_functions_to_subcolumns`. [#31261](https://github.com/ClickHouse/ClickHouse/pull/31261) ([Anton Popov](https://github.com/CurtizJ)).
- Optimize function `mapContains` to reading of subcolumn `key` with enabled settings `optimize_functions_to_subcolumns`. [#31218](https://github.com/ClickHouse/ClickHouse/pull/31218) ([Anton Popov](https://github.com/CurtizJ)).
- Add settings `merge_tree_min_rows_for_concurrent_read_for_remote_filesystem` and `merge_tree_min_bytes_for_concurrent_read_for_remote_filesystem`. [#30970](https://github.com/ClickHouse/ClickHouse/pull/30970) ([Kseniia Sumarokova](https://github.com/kssenii)).
- Skipping mutations of different partitions in `StorageMergeTree`. [#21326](https://github.com/ClickHouse/ClickHouse/pull/21326) ([Vladimir Chebotarev](https://github.com/excitoon)).

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