Use case:
See #21187
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    M(Bool, allow_suspicious_low_cardinality_types, false, "In CREATE TABLE statement allows specifying LowCardinality modifier for types of small fixed size (8 or less). Enabling this may increase merge times and memory consumption.", 0) \
    M(Bool, compile_expressions, false, "Compile some scalar functions and operators to native code.", 0) \
    M(UInt64, min_count_to_compile_expression, 3, "The number of identical expressions before they are JIT-compiled", 0) \
    M(UInt64, group_by_two_level_threshold, 100000, "From what number of keys, a two-level aggregation starts. 0 - the threshold is not set.", 0) \
    M(UInt64, group_by_two_level_threshold_bytes, 100000000, "From what size of the aggregation state in bytes, a two-level aggregation begins to be used. 0 - the threshold is not set. Two-level aggregation is used when at least one of the thresholds is triggered.", 0) \
    M(Bool, distributed_aggregation_memory_efficient, true, "Is the memory-saving mode of distributed aggregation enabled.", 0) \
    M(UInt64, aggregation_memory_efficient_merge_threads, 0, "Number of threads to use for merge intermediate aggregation results in memory efficient mode. When bigger, then more memory is consumed. 0 means - same as 'max_threads'.", 0) \
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    M(UInt64, max_parallel_replicas, 1, "The maximum number of replicas of each shard used when the query is executed. For consistency (to get different parts of the same partition), this option only works for the specified sampling key. The lag of the replicas is not controlled.", 0) \
    M(UInt64, parallel_replicas_count, 0, "", 0) \
    M(UInt64, parallel_replica_offset, 0, "", 0) \
    \
    M(Bool, skip_unavailable_shards, false, "If 1, ClickHouse silently skips unavailable shards and nodes unresolvable through DNS. Shard is marked as unavailable when none of the replicas can be reached.", 0) \
    \
    M(UInt64, parallel_distributed_insert_select, 0, "Process distributed INSERT SELECT query in the same cluster on local tables on every shard, if 1 SELECT is executed on each shard, if 2 SELECT and INSERT is executed on each shard", 0) \
    M(UInt64, distributed_group_by_no_merge, 0, "If 1, Do not merge aggregation states from different servers for distributed query processing - in case it is for certain that there are different keys on different shards. If 2 - same as 1 but also apply ORDER BY and LIMIT stages", 0) \
    M(Bool, optimize_distributed_group_by_sharding_key, false, "Optimize GROUP BY sharding_key queries (by avoiding costly aggregation on the initiator server).", 0) \
    M(UInt64, optimize_skip_unused_shards_limit, 1000, "Limit for number of sharding key values, turns off optimize_skip_unused_shards if the limit is reached", 0) \
    M(Bool, optimize_skip_unused_shards, false, "Assumes that data is distributed by sharding_key. Optimization to skip unused shards if SELECT query filters by sharding_key.", 0) \
    M(Bool, optimize_skip_unused_shards_rewrite_in, true, "Rewrite IN in query for remote shards to exclude values that does not belong to the shard (requires optimize_skip_unused_shards)", 0) \
    M(Bool, allow_nondeterministic_optimize_skip_unused_shards, false, "Allow non-deterministic functions (includes dictGet) in sharding_key for optimize_skip_unused_shards", 0) \
    M(UInt64, force_optimize_skip_unused_shards, 0, "Throw an exception if unused shards cannot be skipped (1 - throw only if the table has the sharding key, 2 - always throw.", 0) \
    M(UInt64, optimize_skip_unused_shards_nesting, 0, "Same as optimize_skip_unused_shards, but accept nesting level until which it will work.", 0) \
    M(UInt64, force_optimize_skip_unused_shards_nesting, 0, "Same as force_optimize_skip_unused_shards, but accept nesting level until which it will work.", 0) \
    \
    M(Bool, input_format_parallel_parsing, true, "Enable parallel parsing for some data formats.", 0) \
    M(UInt64, min_chunk_bytes_for_parallel_parsing, (10 * 1024 * 1024), "The minimum chunk size in bytes, which each thread will parse in parallel.", 0) \
    M(Bool, output_format_parallel_formatting, true, "Enable parallel formatting for some data formats.", 0) \
    \