### ClickHouse release v21.2.6.1-stable FIXME as compared to v21.2.5.5-stable

#### Bug Fix
* Backported in [#21352](https://github.com/ClickHouse/ClickHouse/issues/21352): Fix the number of threads for scalar subqueries and subqueries for index (after [#19007](https://github.com/ClickHouse/ClickHouse/issues/19007) single thread was always used). Fixes [#20457](https://github.com/ClickHouse/ClickHouse/issues/20457), [#20512](https://github.com/ClickHouse/ClickHouse/issues/20512). [#20550](https://github.com/ClickHouse/ClickHouse/pull/20550) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).
* Backported in [#21262](https://github.com/ClickHouse/ClickHouse/issues/21262): fix default_replica_path and default_replica_name values are useless on Replicated(*)MergeTree engine when the engine needs specify other parameters. [#21060](https://github.com/ClickHouse/ClickHouse/pull/21060) ([mxzlxy](https://github.com/mxzlxy)).
* Backported in [#21156](https://github.com/ClickHouse/ClickHouse/issues/21156): fix bug related to cast tuple to map. Closes [#21029](https://github.com/ClickHouse/ClickHouse/issues/21029). [#21120](https://github.com/ClickHouse/ClickHouse/pull/21120) ([hexiaoting](https://github.com/hexiaoting)).
* Backported in [#21234](https://github.com/ClickHouse/ClickHouse/issues/21234): Now mutations allowed only for table engines that support them (MergeTree family, Memory, MaterializedView). Other engines will report a more clear error. Fixes [#21168](https://github.com/ClickHouse/ClickHouse/issues/21168). [#21183](https://github.com/ClickHouse/ClickHouse/pull/21183) ([alesapin](https://github.com/alesapin)).
* Backported in [#21427](https://github.com/ClickHouse/ClickHouse/issues/21427): Fix crash in `EXPLAIN` for query with `UNION`. Fixes [#20876](https://github.com/ClickHouse/ClickHouse/issues/20876), [#21170](https://github.com/ClickHouse/ClickHouse/issues/21170). [#21246](https://github.com/ClickHouse/ClickHouse/pull/21246) ([flynn](https://github.com/ucasfl)).
* Backported in [#21301](https://github.com/ClickHouse/ClickHouse/issues/21301): Fix redundant reconnects to ZooKeeper and the possibility of two active sessions for a single clickhouse server. Both problems introduced in [#14678](https://github.com/ClickHouse/ClickHouse/issues/14678). [#21264](https://github.com/ClickHouse/ClickHouse/pull/21264) ([alesapin](https://github.com/alesapin)).
* Backported in [#21553](https://github.com/ClickHouse/ClickHouse/issues/21553): Now `ALTER MODIFY COLUMN` queries will correctly affect changes in partition key, skip indices, TTLs, and so on. Fixes [#13675](https://github.com/ClickHouse/ClickHouse/issues/13675). [#21334](https://github.com/ClickHouse/ClickHouse/pull/21334) ([alesapin](https://github.com/alesapin)).
* Backported in [#21380](https://github.com/ClickHouse/ClickHouse/issues/21380): Fix error `Bad cast from type ... to DB::ColumnLowCardinality` while inserting into table with `LowCardinality` column from `Values` format. Fixes [#21140](https://github.com/ClickHouse/ClickHouse/issues/21140). [#21357](https://github.com/ClickHouse/ClickHouse/pull/21357) ([Nikolai Kochetov](https://github.com/KochetovNicolai)).