Aiven for ClickHouse® 26.3 default settings

Aiven for ClickHouse® uses a managed configuration that differs from upstream ClickHouse defaults. These differences help keep services reliable, secure, and predictable in Aiven-managed environments.

Aiven applies conservative defaults and constraints where upstream behavior can affect query correctness, upgrade compatibility, or shared resource isolation.

The following sections list the settings where Aiven defaults or constraints differ from defaults in version 26.3, grouped by session, table, and server scope.

How to read the settings tables

Each setting includes an Aiven default value and indicates whether you can change it:

• Can be changed: No: Aiven manages the setting value. You cannot override it.
• Can be changed: Yes: Aiven sets the default value, but you can change it where the setting is configurable.

Where Aiven defaults vary by service plan or node resources, the Aiven default column shows ranges or placeholders. The Description column explains why the default differs or lists any configurable bounds, such as an allowed range or minimum value.

Values shown as ranges or placeholders, such as [3..80, depending on CPU count], {cpu_count}, or [~7% of RAM], are sized automatically based on your service plan and node resources.

note

For configurable settings and Aiven-defined limits, see Advanced parameters for Aiven for ClickHouse® and Limits and limitations.

Session settings

These settings apply to sessions and queries.

Setting	Aiven default	Can be changed	Description
allow_deprecated_error_prone_window_functions	0	No	Keeps deprecated, error-prone window functions disabled.
allow_deprecated_snowflake_conversion_functions	0	No	Keeps deprecated Snowflake conversion functions disabled.
allow_experimental_alias_table_engine	0	No	Prevents use of an experimental table engine with known data-loss and upgrade-compatibility risks.
allow_experimental_database_paimon_rest_catalog	0	No	Prevents unsupported external catalog metadata and credentials from creating security, upgrade, and restore risks.
allow_experimental_kafka_offsets_storage_in_keeper	0	No	Keeps experimental Kafka offset storage in Keeper disabled.
allow_experimental_nullable_tuple_type	0	No	Prevents persisted Nullable(Tuple) data that ClickHouse 25.8 cannot read during rollback or restore.
allow_experimental_object_storage_queue_hive_partitioning	0	No	Prevents experimental per-partition Keeper state from affecting coordination, upgrade, and restore reliability.
allow_experimental_time_series_aggregate_functions	0	No	Prevents use of experimental time-series aggregate functions until allocation limits are validated.
allow_experimental_ts_to_grid_aggregate_function	0	No	Prevents use of the experimental time-series aggregate function alias until allocation limits are validated.
allow_fuzz_query_functions	0	No	Keeps test-only query mutation functions from consuming shared resources or destabilizing the service.
allow_introspection_functions	0	Yes	Keeps sensitive debugging and introspection functions disabled by default.
allow_non_default_profile	0	No	Prevents switching to unmanaged profiles.
ast_fuzzer_any_query	0	No	Prevents the test-only AST fuzzer from mutating write and DDL queries.
ast_fuzzer_runs	0	No	Prevents test-only randomized query execution from consuming resources or destabilizing the service.
cancel_http_readonly_queries_on_client_close	1	Yes	Cancels read-only HTTP queries when clients disconnect.
check_named_collection_dependencies	1	No	Prevents removal of named collections that tables use, protecting table availability and managed backup and restore operations.
cluster_function_process_archive_on_multiple_nodes	0	Yes	Disables archive processing on multiple nodes by default.
compatibility	""	Yes	Does not set an older ClickHouse compatibility version by default.
correlated_subqueries_use_in_memory_buffer	0	Yes	Preserves compatibility with join algorithms and prevents valid correlated subqueries from failing.
database_replicated_allow_replicated_engine_arguments	1	No	Supports replicated database and table creation in Aiven-managed services.
delta_lake_snapshot_end_version	-1	No	Prevents incorrect Delta Lake change data feed results because version 26.3 can silently ignore explicit end bounds on some storage backends.
delta_lake_snapshot_start_version	-1	No	Prevents incorrect Delta Lake change data feed results because version 26.3 can silently ignore explicit start bounds on some storage backends.
distributed_ddl_entry_format_version	7	No	Pins the distributed DDL entry format to avoid inconsistencies during version upgrades.
distributed_ddl_output_mode	null_status_on_timeout	Yes	Makes distributed DDL timeout reporting more resilient to recoverable errors.
enable_materialized_cte	0	No	Keeps experimental materialized common table expressions disabled to prevent known crashes and incorrect dependency ordering.
enable_producing_buckets_out_of_order_in_aggregation	0	No	Keeps aggregation buckets ordered to prevent failures in multi-layer distributed aggregations.
format_display_secrets_in_show_and_select	0	Yes	Avoids exposing secrets in SHOW and SELECT output by default.
http_max_request_header_size	131072	No	Limits HTTP request headers to 128 KiB, protecting service availability by bounding memory allocated before authentication.
http_write_exception_in_output_format	0	Yes	Keeps exceptions out of the requested output format so client errors are easier to detect.
max_autoincrement_series	1000	No	Limits automatic sequence expansion.
max_concurrent_queries_for_all_users	[100..800, depending on service size]	No	Enforces a service-wide concurrency limit.
max_http_get_redirects	10	Yes	Limits HTTP redirect following.
max_insert_threads	2	Yes	Allowed range: 1..{cpu_count}. Caps insert parallelism so a single query does not use too many shared service resources.
max_threads	{cpu_count}	Yes	Allowed range: 1..{cpu_count}. Caps general query parallelism to the node CPU count.
memory_profiler_sample_probability	0	No	Disables random memory profiler sampling.
memory_profiler_step	4194304	Yes	Minimum value: 100000. Keeps the upstream memory profiler granularity while preventing very low values.
min_free_disk_ratio_to_perform_insert	0.05	No	Keeps a disk headroom guard before accepting inserts.
optimize_qbit_distance_function_reads	0	Yes	Disables partial reads that can fail queries using nullable, Variant, or Dynamic vectors.
os_thread_priority	0	Yes	Allowed range: 0..19. Allows normal or lower query priority while preventing workloads from taking priority over shared service operations.
os_threads_nice_value_materialized_view	0	Yes	Allowed range: 0..19. Allows normal or lower materialized-view priority while protecting shared service operations.
os_threads_nice_value_query	0	Yes	Allowed range: 0..19. Allows normal or lower query priority while protecting shared service operations.
output_format_json_quote_64bit_integers	1	Yes	Keeps JSON output compatible with clients that cannot safely represent 64-bit integers.
postgresql_connection_attempt_timeout	10	Yes	Limits each PostgreSQL connection attempt to 10 seconds.
postgresql_connection_pool_connect_timeout	10	Yes	Limits pooled PostgreSQL connection setup to 10 seconds.
push_external_roles_in_interserver_queries	0	No	External authorization is not supported. Roles must be synchronized between nodes.
query_profiler_cpu_time_period_ns	1000000000	Yes	Minimum value: 1000000. Keeps the upstream CPU profiler sampling period while preventing sub-millisecond values.
query_profiler_real_time_period_ns	1000000000	Yes	Minimum value: 1000000. Keeps the upstream real-time profiler sampling period while preventing sub-millisecond values.
query_plan_convert_any_join_to_semi_or_anti_join	0	Yes	Disables a join optimization that can return incorrect results when a dependent set is not ready.
query_plan_direct_read_from_text_index	0	Yes	Disables direct text-index reads because version 26.3 can return incorrect results for nullable columns and updated parts.
query_plan_remove_unused_columns	0	Yes	Preserves earlier query-plan behavior and avoids excessive memory use for aggregate queries with PREWHERE filters over wide string columns.
readonly	0	Yes	Controls read and write access for the session. You can change the value from 0 to 1, but not from 1 to 0.
s3queue_keeper_fault_injection_probability	0	No	Prevents test-only fault injection from disrupting S3Queue ingestion and managed Keeper operations.
stream_like_engine_allow_direct_select	1	Yes	Allows direct SELECT queries on stream-like engines for compatibility with existing workloads.
write_full_path_in_iceberg_metadata	1	Yes	Writes full Iceberg metadata paths for compatibility with mainstream ClickHouse behavior.

Replicated MergeTree table settings

These settings apply to the ReplicatedMergeTree table engine family. On the Aiven platform, MergeTree engines are remapped to their ReplicatedMergeTree variants. See Supported table engines.

Setting	Aiven default	Can be changed	Description
allow_remote_fs_zero_copy_replication	1	No	Enables zero-copy replication for remote filesystem storage. Aiven manages this storage behavior.
disable_detach_partition_for_zero_copy_replication	0	No	Keeps DETACH PARTITION available for zero-copy replicated tables.
disable_fetch_partition_for_zero_copy_replication	0	No	Keeps FETCH PARTITION available for zero-copy replicated tables.
disable_freeze_partition_for_zero_copy_replication	0	No	Keeps FREEZE PARTITION available for zero-copy replicated tables.
enable_max_bytes_limit_for_min_age_to_force_merge	1	No	Enforces the byte-size limit when age-based forced merges are considered. This helps prevent creation of very large parts.
finished_mutations_to_keep	10	Yes	Limits retained finished mutation metadata so ZooKeeper or Keeper state does not grow unnecessarily.
max_parts_in_total	10000	Yes	Allowed range: 100..50000. Sets a cap on table part count to protect merge performance and metadata size.
number_of_free_entries_in_pool_to_execute_mutation	20	Yes	Uses the upstream default while limiting the value below the service background pool capacity.
number_of_free_entries_in_pool_to_execute_optimize_entire_partition	25	Yes	Uses the upstream default while limiting the value below the service background pool capacity.
number_of_free_entries_in_pool_to_lower_max_size_of_merge	8	Yes	Uses the upstream default while limiting the value below the service background pool capacity.
old_parts_lifetime	60	Yes	Reduces the lifetime of outdated merged parts to lower the number of ZooKeeper or Keeper metadata nodes.
serialization_info_version	basic	Yes	Preserves part compatibility with ClickHouse 25.8 during rolling upgrades, rollback, and restore operations. Keep this value until all nodes run 26.3.
table_readonly	0	No	Keeps an unsupported mode disabled because it can block managed operations and distributed DDL without protecting replicated tables.

Server settings

These settings are configured at the server level and are managed by Aiven. They are not configurable as session settings. Most settings are derived from your service plan. Some settings, such as vector_similarity_index_cache_size, are exposed through advanced configuration.

Setting	Aiven default	Description
aiven_enable_replication_queue_size_limit	1	Preserves replication queue limits that delay or reject inserts before queue growth affects service availability.
aiven_enforce_default_replication_path	1	Keeps replicated tables in Aiven-managed Keeper paths to preserve service isolation and reliable lifecycle operations.
aiven_prohibit_tmp_table_creation	1	Reserves internal .tmp table names so your DDL cannot interfere with managed table operations.
aiven_replace_mergetree_with_replicated	1	Converts MergeTree engines to replicated variants in Replicated databases, preserving managed high availability.
aiven_skip_azure_container_creation	1	Skips Azure container probing during DDL replay so backup and restore can proceed without weakening data-access authentication.
background_pool_size	[8..32, depending on CPU count]	Sizes the background merge and mutation pool based on service CPU capacity. If an explicit service-level override is configured, Aiven preserves it.
background_schedule_pool_size	[24..512, depending on CPU count]	Scales lightweight background scheduling with service CPU capacity and avoids excessive idle threads on smaller plans.
cgroups_memory_usage_observer_wait_time	0	Disables the ClickHouse cgroup memory usage observer because Aiven sets explicit memory limits for the managed server process.
cluster_database	default	Sets the database used by cluster-related helpers to the managed default database.
database_atomic_delay_before_drop_table_sec	0	Removes the Atomic database delayed-drop wait to prevent race conditions in refreshable materialized views.
database_replicated.internal_replication	1	Routes Distributed inserts once per shard and lets ReplicatedMergeTree handle replica distribution, preventing duplicate writes.
default_database	default	Uses the managed default database unless another database is selected explicitly.
default_replica_name	{replica}	Uses the Aiven-provided replica macro in replicated table paths.
dictionary_user	avnadmin	Runs dictionary queries through the managed admin user.
disable_internal_dns_cache	1	Prevents ClickHouse from keeping stale internal DNS results in a managed environment where node addresses can change.
display_secrets_in_show_and_select	1	Keeps server-level secret display available for privileged managed operations. User-facing profile settings hide secrets by default.
enforce_https_for_url_storage	1	Requires encrypted transport for URL storage sources and HTTP dictionaries.
iceberg_catalog_threadpool_pool_size	[3..80, depending on CPU count]	Scales Iceberg catalog worker concurrency with service CPU capacity.
iceberg_metadata_files_cache_size	[~7% of RAM]	Sizes the Iceberg metadata file cache based on the managed memory budget.
index_mark_cache_size	[~7% of RAM]	Sizes the secondary index mark cache based on the managed memory budget.
keep_alive_timeout	10	Limits idle keep-alive connections to 10 seconds so clients cannot hold server resources indefinitely.
load_marks_threadpool_pool_size	[3..80, depending on CPU count]	Scales mark-loading concurrency with service CPU capacity.
mark_cache_size	[~7% of RAM]	Sizes the primary mark cache based on the managed memory budget.
max_concurrent_queries	[147..847, depending on service size]	Sets a server-wide concurrency limit based on the service plan, with extra internal capacity reserved for monitoring, backup, and operator queries.
max_connections	[1000..4000, depending on service size]	Sets the connection limit based on service size. Smaller nodes have lower limits to prevent overload, and larger service plans allow higher limits.
max_database_num_to_throw	400	Enforces a hard limit on the number of databases to protect metadata and reduce operational overhead.
max_database_num_to_warn	200	Warns before the hard database-count limit is reached.
max_format_parsing_thread_pool_size	[3..80, depending on CPU count]	Scales the format parsing thread pool with service CPU capacity.
max_named_collection_num_to_throw	5000	Limits user-managed named collections to 5,000 to protect Keeper metadata, configuration reloads, and backup operations.
max_partition_size_to_drop	0	Removes the server-side size limit on partition drops so you can drop large partitions.
max_prefixes_deserialization_thread_pool_size	[3..80, depending on CPU count]	Scales the prefixes deserialization thread pool with service CPU capacity.
max_server_memory_usage	[65%..70% of host RAM, depending on service size]	Caps ClickHouse memory use below total host RAM so the node retains memory for the operating system, caching, and service management operations.
max_table_size_to_drop	0	Removes the server-side size limit on table drops so you can drop large tables.
memory_worker_correct_memory_tracker	1	Lets the ClickHouse memory worker correct memory tracking drift.
mysql_require_secure_transport	1	Rejects plaintext MySQL protocol connections to protect credentials and query traffic.
parquet_metadata_cache_size	[~7% of RAM]	Scales the Parquet metadata cache with service memory to prevent disproportionate use on smaller plans.
postgresql_require_secure_transport	1	Rejects plaintext PostgreSQL protocol connections to protect credentials and query traffic.
prefetch_threadpool_pool_size	[3..80, depending on CPU count]	Scales the prefetch thread pool with service CPU capacity.
prepare_system_log_tables_on_startup	1	Ensures system log tables are ready immediately after startup for managed observability and diagnostics.
reserved_replicated_database_prefixes	["aiven", "endpoint_", "service_"]	Reserves internal database name prefixes for Aiven-managed objects.
series_keeper_path	/clickhouse/series	Sets the Keeper path for generateSerialID counter nodes.
show_addresses_in_stack_traces	0	Avoids exposing raw addresses in stack traces returned to users or logs.
text_index_header_cache_size	[~1.75% of RAM, maximum 1 GiB]	Assigns 25% of a plan-scaled text-index cache budget to headers, protecting smaller services from disproportionate memory use.
text_index_postings_cache_size	[~3.5% of RAM, maximum 2 GiB]	Assigns 50% of a plan-scaled text-index cache budget to posting lists, protecting smaller services from disproportionate memory use.
text_index_tokens_cache_size	[~1.75% of RAM, maximum 1 GiB]	Assigns 25% of a plan-scaled text-index cache budget to tokens, protecting smaller services from disproportionate memory use.
threadpool_local_fs_reader_pool_size	[3..80, depending on CPU count]	Scales local filesystem reader concurrency with service CPU capacity.
threadpool_remote_fs_reader_pool_size	[3..80, depending on CPU count]	Scales remote filesystem reader concurrency with service CPU capacity.
threadpool_writer_pool_size	[3..80, depending on CPU count]	Scales filesystem writer concurrency with service CPU capacity.
uncompressed_cache_size	[~7% of RAM]	Sizes the uncompressed block cache based on the managed memory budget.
user_with_indirect_database_creation	avnadmin	Restricts indirect database creation privileges to the managed admin user.
vector_similarity_index_cache_size	0.07	Sizes the vector similarity index cache to 7% of server memory. Set to 0 to disable the cache. Maximum value: 0.5.