title_wip

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

- Support deduplication for asynchronous inserts. Before this change, async inserts did not support deduplication, because multiple small inserts coexisted in one inserted batch. Closes [#38075](https://github.com/ClickHouse/ClickHouse/issues/38075). [#43304](https://github.com/ClickHouse/ClickHouse/pull/43304) ([Han Fei](https://github.com/hanfei1991)).
- Add support for cosine distance for the experimental Annoy (vector similarity search) index. [#42778](https://github.com/ClickHouse/ClickHouse/pull/42778) ([Filatenkov Artur](https://github.com/FArthur-cmd)).
- Add `CREATE / ALTER / DROP NAMED COLLECTION` queries. [#43252](https://github.com/ClickHouse/ClickHouse/pull/43252) ([Kseniia Sumarokova](https://github.com/kssenii)). This feature is under development and the queries are not effective as of version 22.12. This changelog entry is added only to avoid confusion. Restrict default access to named collections to the user defined in config. This requires that `show_named_collections = 1` is set to be able to see them. [#43325](https://github.com/ClickHouse/ClickHouse/pull/43325) ([Kseniia Sumarokova](https://github.com/kssenii)). The `system.named_collections` table is introduced [#43147](https://github.com/ClickHouse/ClickHouse/pull/43147) ([Kseniia Sumarokova](https://github.com/kssenii)).

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

- Add settings `max_streams_for_merge_tree_reading` and `allow_asynchronous_read_from_io_pool_for_merge_tree`. Setting `max_streams_for_merge_tree_reading` limits the number of reading streams for MergeTree tables. Setting `allow_asynchronous_read_from_io_pool_for_merge_tree` enables a background I/O pool to read from `MergeTree` tables. This may increase performance for I/O bound queries if used together with `max_streams_to_max_threads_ratio` or `max_streams_for_merge_tree_reading`. [#43260](https://github.com/ClickHouse/ClickHouse/pull/43260) ([Nikolai Kochetov](https://github.com/KochetovNicolai)). This improves performance up to 100 times in case of high latency storage, low number of CPU and high number of data parts.
- Settings `merge_tree_min_rows_for_concurrent_read_for_remote_filesystem/merge_tree_min_bytes_for_concurrent_read_for_remote_filesystem` did not respect adaptive granularity. Fat rows did not decrease the number of read rows (as it was done for `merge_tree_min_rows_for_concurrent_read/merge_tree_min_bytes_for_concurrent_read`, which could lead to high memory usage when using remote filesystems. [#43965](https://github.com/ClickHouse/ClickHouse/pull/43965) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).
- Optimized the number of list requests to ZooKeeper or ClickHouse Keeper when selecting a part to merge. Previously it could produce thousands of requests in some cases. Fixes [#43647](https://github.com/ClickHouse/ClickHouse/issues/43647). [#43675](https://github.com/ClickHouse/ClickHouse/pull/43675) ([Alexander Tokmakov](https://github.com/tavplubix)).
- Optimization is getting skipped now if `max_size_to_preallocate_for_aggregation` has too small a value. The default value of this setting increased to `10^8`. [#43945](https://github.com/ClickHouse/ClickHouse/pull/43945) ([Nikita Taranov](https://github.com/nickitat)).
- Speed-up server shutdown by avoiding cleaning up of old data parts. Because it is unnecessary after <https://github.com/ClickHouse/ClickHouse/pull/41145>. [#43760](https://github.com/ClickHouse/ClickHouse/pull/43760) ([Sema Checherinda](https://github.com/CheSema)).
- Merging on initiator now uses the same memory bound approach as merging of local aggregation results if `enable_memory_bound_merging_of_aggregation_results` is set. [#40879](https://github.com/ClickHouse/ClickHouse/pull/40879) ([Nikita Taranov](https://github.com/nickitat)).
- Keeper improvement: try syncing logs to disk in parallel with replication. [#43450](https://github.com/ClickHouse/ClickHouse/pull/43450) ([Antonio Andelic](https://github.com/antonio2368)).
- Keeper improvement: requests are batched more often. The batching can be controlled with the new setting `max_requests_quick_batch_size`. [#43686](https://github.com/ClickHouse/ClickHouse/pull/43686) ([Antonio Andelic](https://github.com/antonio2368)).

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