title_wip

#### Performance Improvement[​](#performance-improvement-11 "Direct link to Performance Improvement")

- Support moving conditions to `PREWHERE` (setting `optimize_move_to_prewhere`) for tables of `Merge` engine if its all underlying tables supports `PREWHERE`. [#33300](https://github.com/ClickHouse/ClickHouse/pull/33300) ([Anton Popov](https://github.com/CurtizJ)).
- More efficient handling of globs for URL storage. Now you can easily query million URLs in parallel with retries. Closes [#32866](https://github.com/ClickHouse/ClickHouse/issues/32866). [#32907](https://github.com/ClickHouse/ClickHouse/pull/32907) ([Kseniia Sumarokova](https://github.com/kssenii)).
- Avoid exponential backtracking in parser. This closes [#20158](https://github.com/ClickHouse/ClickHouse/issues/20158). [#33481](https://github.com/ClickHouse/ClickHouse/pull/33481) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Abuse of `untuple` function was leading to exponential complexity of query analysis (found by fuzzer). This closes [#33297](https://github.com/ClickHouse/ClickHouse/issues/33297). [#33445](https://github.com/ClickHouse/ClickHouse/pull/33445) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Reduce allocated memory for dictionaries with string attributes. [#33466](https://github.com/ClickHouse/ClickHouse/pull/33466) ([Maksim Kita](https://github.com/kitaisreal)).
- Slight performance improvement of `reinterpret` function. [#32587](https://github.com/ClickHouse/ClickHouse/pull/32587) ([alexey-milovidov](https://github.com/alexey-milovidov)).
- Non significant change. In extremely rare cases when data part is lost on every replica, after merging of some data parts, the subsequent queries may skip less amount of partitions during partition pruning. This hardly affects anything. [#32220](https://github.com/ClickHouse/ClickHouse/pull/32220) ([Azat Khuzhin](https://github.com/azat)).
- Improve `clickhouse-keeper` writing performance by optimization the size calculation logic. [#32366](https://github.com/ClickHouse/ClickHouse/pull/32366) ([zhanglistar](https://github.com/zhanglistar)).
- Optimize single part projection materialization. This closes [#31669](https://github.com/ClickHouse/ClickHouse/issues/31669). [#31885](https://github.com/ClickHouse/ClickHouse/pull/31885) ([Amos Bird](https://github.com/amosbird)).
- Improve query performance of system tables. [#33312](https://github.com/ClickHouse/ClickHouse/pull/33312) ([OnePiece](https://github.com/zhongyuankai)).
- Optimize selecting of MergeTree parts that can be moved between volumes. [#33225](https://github.com/ClickHouse/ClickHouse/pull/33225) ([OnePiece](https://github.com/zhongyuankai)).
- Fix `sparse_hashed` dict performance with sequential keys (wrong hash function). [#32536](https://github.com/ClickHouse/ClickHouse/pull/32536) ([Azat Khuzhin](https://github.com/azat)).

#### Experimental Feature[​](#experimental-feature-10 "Direct link to Experimental Feature")

- Parallel reading from multiple replicas within a shard during distributed query without using sample key. To enable this, set `allow_experimental_parallel_reading_from_replicas = 1` and `max_parallel_replicas` to any number. This closes [#26748](https://github.com/ClickHouse/ClickHouse/issues/26748). [#29279](https://github.com/ClickHouse/ClickHouse/pull/29279) ([Nikita Mikhaylov](https://github.com/nikitamikhaylov)).
- Implemented sparse serialization. It can reduce usage of disk space and improve performance of some queries for columns, which contain a lot of default (zero) values. It can be enabled by setting `ratio_for_sparse_serialization`. Sparse serialization will be chosen dynamically for column, if it has ratio of number of default values to number of all values above that threshold. Serialization (default or sparse) will be fixed for every column in part, but may varies between parts. [#22535](https://github.com/ClickHouse/ClickHouse/pull/22535) ([Anton Popov](https://github.com/CurtizJ)).
- Add "TABLE OVERRIDE" feature for customizing MaterializedMySQL table schemas. [#32325](https://github.com/ClickHouse/ClickHouse/pull/32325) ([Stig Bakken](https://github.com/stigsb)).
- Add `EXPLAIN TABLE OVERRIDE` query. [#32836](https://github.com/ClickHouse/ClickHouse/pull/32836) ([Stig Bakken](https://github.com/stigsb)).
- Support TABLE OVERRIDE clause for MaterializedPostgreSQL. RFC: [#31480](https://github.com/ClickHouse/ClickHouse/issues/31480). [#32749](https://github.com/ClickHouse/ClickHouse/pull/32749) ([Kseniia Sumarokova](https://github.com/kssenii)).
- Change ZooKeeper path for zero-copy marks for shared data. Note that "zero-copy replication" is non-production feature (in early stages of development) that you shouldn't use anyway. But in case if you have used it, let you keep in mind this change. [#32061](https://github.com/ClickHouse/ClickHouse/pull/32061) ([ianton-ru](https://github.com/ianton-ru)).
- Events clause support for WINDOW VIEW watch query. [#32607](https://github.com/ClickHouse/ClickHouse/pull/32607) ([vxider](https://github.com/Vxider)).
- Fix ACL with explicit digit hash in `clickhouse-keeper`: now the behavior consistent with ZooKeeper and generated digest is always accepted. [#33249](https://github.com/ClickHouse/ClickHouse/pull/33249) ([小路](https://github.com/nicelulu)). [#33246](https://github.com/ClickHouse/ClickHouse/pull/33246).
- Fix unexpected projection removal when detaching parts. [#32067](https://github.com/ClickHouse/ClickHouse/pull/32067) ([Amos Bird](https://github.com/amosbird)).

#### Improvement[​](#improvement-11 "Direct link to Improvement")