2021 Changelog

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

- Hedged requests for remote queries. When setting `use_hedged_requests` enabled (off by default), allow to establish many connections with different replicas for query. New connection is enabled in case existent connection(s) with replica(s) were not established within `hedged_connection_timeout` or no data was received within `receive_data_timeout`. Query uses the first connection which send non empty progress packet (or data packet, if `allow_changing_replica_until_first_data_packet`); other connections are cancelled. Queries with `max_parallel_replicas > 1` are supported. [#19291](https://github.com/ClickHouse/ClickHouse/pull/19291) ([Kruglov Pavel](https://github.com/Avogar)). This allows to significantly reduce tail latencies on very large clusters.
- Added support for `PREWHERE` (and enable the corresponding optimization) when tables have row-level security expressions specified. [#19576](https://github.com/ClickHouse/ClickHouse/pull/19576) ([Denis Glazachev](https://github.com/traceon)).
- The setting `distributed_aggregation_memory_efficient` is enabled by default. It will lower memory usage and improve performance of distributed queries. [#20599](https://github.com/ClickHouse/ClickHouse/pull/20599) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Improve performance of GROUP BY multiple fixed size keys. [#20472](https://github.com/ClickHouse/ClickHouse/pull/20472) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Improve performance of aggregate functions by more strict aliasing. [#19946](https://github.com/ClickHouse/ClickHouse/pull/19946) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Speed up reading from `Memory` tables in extreme cases (when reading speed is in order of 50 GB/sec) by simplification of pipeline and (consequently) less lock contention in pipeline scheduling. [#20468](https://github.com/ClickHouse/ClickHouse/pull/20468) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Partially reimplement HTTP server to make it making less copies of incoming and outgoing data. It gives up to 1.5 performance improvement on inserting long records over HTTP. [#19516](https://github.com/ClickHouse/ClickHouse/pull/19516) ([Ivan](https://github.com/abyss7)).
- Add `compress` setting for `Memory` tables. If it's enabled the table will use less RAM. On some machines and datasets it can also work faster on SELECT, but it is not always the case. This closes [#20093](https://github.com/ClickHouse/ClickHouse/issues/20093). Note: there are reasons why Memory tables can work slower than MergeTree: (1) lack of compression (2) static size of blocks (3) lack of indices and prewhere... [#20168](https://github.com/ClickHouse/ClickHouse/pull/20168) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Slightly better code in aggregation. [#20978](https://github.com/ClickHouse/ClickHouse/pull/20978) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Add back `intDiv`/`modulo` specializations for better performance. This fixes [#21293](https://github.com/ClickHouse/ClickHouse/issues/21293) . The regression was introduced in <https://github.com/ClickHouse/ClickHouse/pull/18145> . [#21307](https://github.com/ClickHouse/ClickHouse/pull/21307) ([Amos Bird](https://github.com/amosbird)).
- Do not squash blocks too much on INSERT SELECT if inserting into Memory table. In previous versions inefficient data representation was created in Memory table after INSERT SELECT. This closes [#13052](https://github.com/ClickHouse/ClickHouse/issues/13052). [#20169](https://github.com/ClickHouse/ClickHouse/pull/20169) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Fix at least one case when DataType parser may have exponential complexity (found by fuzzer). This closes [#20096](https://github.com/ClickHouse/ClickHouse/issues/20096). [#20132](https://github.com/ClickHouse/ClickHouse/pull/20132) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Parallelize SELECT with FINAL for single part with level > 0 when `do_not_merge_across_partitions_select_final` setting is 1. [#19375](https://github.com/ClickHouse/ClickHouse/pull/19375) ([Kruglov Pavel](https://github.com/Avogar)).
- Fill only requested columns when querying `system.parts` and `system.parts_columns`. Closes [#19570](https://github.com/ClickHouse/ClickHouse/issues/19570). [#21035](https://github.com/ClickHouse/ClickHouse/pull/21035) ([Anmol Arora](https://github.com/anmolarora)).
- Perform algebraic optimizations of arithmetic expressions inside `avg` aggregate function. close [#20092](https://github.com/ClickHouse/ClickHouse/issues/20092). [#20183](https://github.com/ClickHouse/ClickHouse/pull/20183) ([flynn](https://github.com/ucasFL)).

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