max_prefixes_deserialization_thread_pool_free_size
If the number of idle threads in the prefixes deserialization Thread pool exceedsmax_prefixes_deserialization_thread_pool_free_size, ClickHouse will release resources occupied by idling threads and decrease the pool size. Threads can be created again if necessary.
max_prefixes_deserialization_thread_pool_size
ClickHouse uses threads from the prefixes deserialization Thread pool for parallel reading of metadata of columns and subcolumns from file prefixes in Wide parts in MergeTree.max_prefixes_deserialization_thread_pool_size limits the maximum number of threads in the pool.