max_threads
The maximum number of query processing threads, excluding threads for retrieving data from remote servers (see the ‘max_distributed_connections’ parameter). This parameter applies to threads that perform the same stages of the query processing pipeline in parallel. For example, when reading from a table, if it is possible to evaluate expressions with functions, filter withWHERE and pre-aggregate for GROUP BY in parallel using at least ‘max_threads’ number of threads, then ‘max_threads’ are used.
For queries that are completed quickly because of a LIMIT, you can set a lower ‘max_threads’.
For example, if the necessary number of entries are located in every block and max_threads = 8, then 8 blocks are retrieved, although it would have been enough to read just one.
The smaller the max_threads value, the less memory is consumed.
The max_threads setting by default matches the number of hardware threads (number of CPU cores) available to ClickHouse.
As a special case, for x86 processors with less than 32 CPU cores and SMT (e.g. Intel HyperThreading), ClickHouse uses the number of logical cores (= 2 x physical core count) by default.
Without SMT (e.g. Intel HyperThreading), this corresponds to the number of CPU cores.
For ClickHouse Cloud users, the default value will display as auto(N) where N matches the vCPU size of your service e.g. 2vCPU/8GiB, 4vCPU/16GiB etc.
See the settings tab in the Cloud console for a list of all service sizes.
max_threads_for_indexes
The maximum number of threads process indices.max_threads_min_free_memory_per_thread
Lowersmax_threads when the server is under memory pressure, to avoid starting highly-parallel queries that are likely to hit the memory limit.
Free memory is computed as the server’s max_server_memory_usage minus the memory currently tracked by the global memory tracker. If that free memory is less than max_threads multiplied by this value, max_threads is reduced to the largest N such that N * value <= free_memory, with a minimum of 1.
Set to 0 to disable this limit.
For example, with the default of 1 GiB and 32 GiB of free memory, max_threads is capped at 32; with 1 GiB of free memory it falls back to 1.
This setting applies to read-side parallelism (SELECT, UNION, INTERSECT/EXCEPT, and the SELECT side of INSERT ... SELECT). For the write side, see max_insert_threads_min_free_memory_per_thread.