2025 Changelog

#### Performance Improvement[​](#performance-improvement-5 "Direct link to Performance Improvement")

- Introduce async logging. When logs are output to a slow device, it no longer delays queries. [#82516](https://github.com/ClickHouse/ClickHouse/pull/82516) ([Raúl Marín](https://github.com/Algunenano)). Limit the max number of entries that are hold in the queue. [#83214](https://github.com/ClickHouse/ClickHouse/pull/83214) ([Raúl Marín](https://github.com/Algunenano)).
- Parallel distributed INSERT SELECT is enabled by default in mode where INSERT SELECT executed on each shard independently, see `parallel_distributed_insert_select` setting. [#83040](https://github.com/ClickHouse/ClickHouse/pull/83040) ([Igor Nikonov](https://github.com/devcrafter)).
- When the aggregation query contains only a single `count()` function on a not-`Nullable` column, the aggregation logic is fully inlined during hash table probing. This avoids allocating and maintaining any aggregation state, significantly reducing memory usage and CPU overhead. This partially addresses [#81982](https://github.com/ClickHouse/ClickHouse/issues/81982). [#82104](https://github.com/ClickHouse/ClickHouse/pull/82104) ([Amos Bird](https://github.com/amosbird)).
- Performance of `HashJoin` optimised by removing the additional loop over hash maps in the typical case of only one key column, also `null_map` and `join_mask` checks are eliminated when they're always `true`/`false`. [#82308](https://github.com/ClickHouse/ClickHouse/pull/82308) ([Nikita Taranov](https://github.com/nickitat)).
- Trivial optimization for `-If` combinator. [#78454](https://github.com/ClickHouse/ClickHouse/pull/78454) ([李扬](https://github.com/taiyang-li)).
- Vector search queries using a vector similarity index complete with lower latency due to reduced storage reads and reduced CPU usage. [#79103](https://github.com/ClickHouse/ClickHouse/pull/79103) ([Shankar Iyer](https://github.com/shankar-iyer)).
- Respect `merge_tree_min_{rows,bytes}_for_seek` in `filterPartsByQueryConditionCache` to align it with other methods filtering by indexes. [#80312](https://github.com/ClickHouse/ClickHouse/pull/80312) ([李扬](https://github.com/taiyang-li)).
- Make the pipeline after the `TOTALS` step multithreaded. [#80331](https://github.com/ClickHouse/ClickHouse/pull/80331) ([UnamedRus](https://github.com/UnamedRus)).
- Fix filter by key for `Redis` and `KeeperMap` storages. [#81833](https://github.com/ClickHouse/ClickHouse/pull/81833) ([Pervakov Grigorii](https://github.com/GrigoryPervakov)).
- Add new setting `min_joined_block_size_rows` (analogous to `min_joined_block_size_bytes`; defaults to 65409) to control the minimum block size (in rows) for JOIN input and output blocks (if the join algorithm supports it). Small blocks will be squashed. [#81886](https://github.com/ClickHouse/ClickHouse/pull/81886) ([Nikita Taranov](https://github.com/nickitat)).
- `ATTACH PARTITION` no longer leads to the dropping of all caches. [#82377](https://github.com/ClickHouse/ClickHouse/pull/82377) ([Alexey Milovidov](https://github.com/alexey-milovidov)).
- Optimize the generated plan for correlated subqueries by removing redundant JOIN operations using equivalence classes. If there are equivalent expressions for all correlated columns, `CROSS JOIN` is not produced if `query_plan_correlated_subqueries_use_substitution` setting is enabled. [#82435](https://github.com/ClickHouse/ClickHouse/pull/82435) ([Dmitry Novik](https://github.com/novikd)).
- Read only required columns in correlated subquery when it appears to be an argument of function `EXISTS`. [#82443](https://github.com/ClickHouse/ClickHouse/pull/82443) ([Dmitry Novik](https://github.com/novikd)).
- Speedup comparisons of query trees during the query analysis a bit. [#82617](https://github.com/ClickHouse/ClickHouse/pull/82617) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).
- Add alignment in the Counter of ProfileEvents to reduce false sharing. [#82697](https://github.com/ClickHouse/ClickHouse/pull/82697) ([Jiebin Sun](https://github.com/jiebinn)).
- The optimizations for `null_map` and `JoinMask` from [#82308](https://github.com/ClickHouse/ClickHouse/issues/82308) were applied to the case of JOIN with multiple disjuncts. Also, the `KnownRowsHolder` data structure was optimized. [#83041](https://github.com/ClickHouse/ClickHouse/pull/83041) ([Nikita Taranov](https://github.com/nickitat)).
- Plain `std::vector<std::atomic_bool>` is used for join flags to avoid calculating a hash on each access to flags. [#83043](https://github.com/ClickHouse/ClickHouse/pull/83043) ([Nikita Taranov](https://github.com/nickitat)).
- Don't pre-allocate memory for result columns beforehand when `HashJoin` uses `lazy` output mode. It is suboptimal, especially when the number of matches is low. Moreover, we know the exact amount of matches after joining is done, so we can preallocate more precisely. [#83304](https://github.com/ClickHouse/ClickHouse/pull/83304) ([Nikita Taranov](https://github.com/nickitat)).
- Minimize memory copy in port headers during pipeline construction. Original [PR](https://github.com/ClickHouse/ClickHouse/pull/70105) by [heymind](https://github.com/heymind). [#83381](https://github.com/ClickHouse/ClickHouse/pull/83381) ([Raúl Marín](https://github.com/Algunenano)).
- Improve the startup of clickhouse-keeper when it uses rocksdb storage. [#83390](https://github.com/ClickHouse/ClickHouse/pull/83390) ([Antonio Andelic](https://github.com/antonio2368)).
- Avoid holding the lock while creating storage snapshot data to reduce lock contention with high concurrent load. [#83510](https://github.com/ClickHouse/ClickHouse/pull/83510) ([Duc Canh Le](https://github.com/canhld94)).
- Improved performance of the `ProtobufSingle` input format by reusing the serializer when no parsing errors occur. [#83613](https://github.com/ClickHouse/ClickHouse/pull/83613) ([Eduard Karacharov](https://github.com/korowa)).
- Improve the performance of pipeline building that speeds up short queries. [#83631](https://github.com/ClickHouse/ClickHouse/pull/83631) ([Raúl Marín](https://github.com/Algunenano)).
- Optimize `MergeTreeReadersChain::getSampleBlock` that speeds up short queries. [#83875](https://github.com/ClickHouse/ClickHouse/pull/83875) ([Raúl Marín](https://github.com/Algunenano)).
- Speedup tables listing in data catalogs by asynchronous requests. [#81084](https://github.com/ClickHouse/ClickHouse/pull/81084) ([alesapin](https://github.com/alesapin)).
- Introduce jitter to the S3 retry mechanism when the `s3_slow_all_threads_after_network_error` configuration is enabled. [#81849](https://github.com/ClickHouse/ClickHouse/pull/81849) ([zoomxi](https://github.com/zoomxi)).

#### Improvement[​](#improvement-5 "Direct link to Improvement")