MergeTree table settings
System table system.merge_tree_settings
shows the globally set MergeTree settings.
MergeTree settings can be set in the merge_tree
section of the server config file, or specified for each MergeTree
table individually in
the SETTINGS
clause of the CREATE TABLE
statement.
Example for customizing setting max_suspicious_broken_parts
:
Configure the default for all MergeTree
tables in the server configuration file:
<merge_tree>
<max_suspicious_broken_parts>5</max_suspicious_broken_parts>
</merge_tree>
Set for a particular table:
CREATE TABLE tab
(
`A` Int64
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS max_suspicious_broken_parts = 500;
Change the settings for a particular table using ALTER TABLE ... MODIFY SETTING
:
ALTER TABLE tab MODIFY SETTING max_suspicious_broken_parts = 100;
-- reset to global default (value from system.merge_tree_settings)
ALTER TABLE tab RESET SETTING max_suspicious_broken_parts;
index_granularity
Maximum number of data rows between the marks of an index.
Default value: 8192.
index_granularity_bytes
Maximum size of data granules in bytes.
Default value: 10Mb.
To restrict the granule size only by number of rows, set to 0 (not recommended).
min_index_granularity_bytes
Min allowed size of data granules in bytes.
Default value: 1024b.
To provide a safeguard against accidentally creating tables with very low index_granularity_bytes.
enable_mixed_granularity_parts
Enables or disables transitioning to control the granule size with the index_granularity_bytes
setting. Before version 19.11, there was only the index_granularity
setting for restricting granule size. The index_granularity_bytes
setting improves ClickHouse performance when selecting data from tables with big rows (tens and hundreds of megabytes). If you have tables with big rows, you can enable this setting for the tables to improve the efficiency of SELECT
queries.
use_minimalistic_part_header_in_zookeeper
Storage method of the data parts headers in ZooKeeper. If enabled, ZooKeeper stores less data. For details, see here.
min_merge_bytes_to_use_direct_io
The minimum data volume for merge operation that is required for using direct I/O access to the storage disk.
When merging data parts, ClickHouse calculates the total storage volume of all the data to be merged.
If the volume exceeds min_merge_bytes_to_use_direct_io
bytes, ClickHouse reads and writes the data to the storage disk using the direct I/O interface (O_DIRECT
option).
If min_merge_bytes_to_use_direct_io = 0
, then direct I/O is disabled.
Default value: 10 * 1024 * 1024 * 1024
bytes.
merge_with_ttl_timeout
Minimum delay in seconds before repeating a merge with delete TTL.
Default value: 14400
seconds (4 hours).
merge_with_recompression_ttl_timeout
Minimum delay in seconds before repeating a merge with recompression TTL.
Default value: 14400
seconds (4 hours).
write_final_mark
Enables or disables writing the final index mark at the end of data part (after the last byte).
Default value: 1.
Don’t change or bad things will happen.
storage_policy
Storage policy.
min_bytes_for_wide_part
Minimum number of bytes/rows in a data part that can be stored in Wide
format.
You can set one, both or none of these settings.
max_compress_block_size
Maximum size of blocks of uncompressed data before compressing for writing to a table. You can also specify this setting in the global settings (see max_compress_block_size setting). The value specified when table is created overrides the global value for this setting.
min_compress_block_size
Minimum size of blocks of uncompressed data required for compression when writing the next mark. You can also specify this setting in the global settings (see min_compress_block_size setting). The value specified when table is created overrides the global value for this setting.
max_suspicious_broken_parts
If the number of broken parts in a single partition exceeds the max_suspicious_broken_parts
value, automatic deletion is denied.
Possible values:
- Any positive integer.
Default value: 100.
parts_to_throw_insert
If the number of active parts in a single partition exceeds the parts_to_throw_insert
value, INSERT
is interrupted with the Too many parts (N). Merges are processing significantly slower than inserts
exception.
Possible values:
- Any positive integer.
Default value: 3000.
To achieve maximum performance of SELECT
queries, it is necessary to minimize the number of parts processed, see Merge Tree.
Prior to 23.6 this setting was set to 300. You can set a higher different value, it will reduce the probability of the Too many parts
error, but at the same time SELECT
performance might degrade. Also in case of a merge issue (for example, due to insufficient disk space) you will notice it later than it could be with the original 300.
parts_to_delay_insert
If the number of active parts in a single partition exceeds the parts_to_delay_insert
value, an INSERT
artificially slows down.
Possible values:
- Any positive integer.
Default value: 1000.
ClickHouse artificially executes INSERT
longer (adds ‘sleep’) so that the background merge process can merge parts faster than they are added.
inactive_parts_to_throw_insert
If the number of inactive parts in a single partition more than the inactive_parts_to_throw_insert
value, INSERT
is interrupted with the "Too many inactive parts (N). Parts cleaning are processing significantly slower than inserts" exception.
Possible values:
- Any positive integer.
Default value: 0 (unlimited).
inactive_parts_to_delay_insert
If the number of inactive parts in a single partition in the table at least that many the inactive_parts_to_delay_insert
value, an INSERT
artificially slows down. It is useful when a server fails to clean up parts quickly enough.
Possible values:
- Any positive integer.
Default value: 0 (unlimited).
max_delay_to_insert
The value in seconds, which is used to calculate the INSERT
delay, if the number of active parts in a single partition exceeds the parts_to_delay_insert value.
Possible values:
- Any positive integer.
Default value: 1.
The delay (in milliseconds) for INSERT
is calculated by the formula:
max_k = parts_to_throw_insert - parts_to_delay_insert
k = 1 + parts_count_in_partition - parts_to_delay_insert
delay_milliseconds = pow(max_delay_to_insert * 1000, k / max_k)
For example, if a partition has 299 active parts and parts_to_throw_insert = 300, parts_to_delay_insert = 150, max_delay_to_insert = 1, INSERT
is delayed for pow( 1 * 1000, (1 + 299 - 150) / (300 - 150) ) = 1000
milliseconds.
Starting from version 23.1 formula has been changed to:
allowed_parts_over_threshold = parts_to_throw_insert - parts_to_delay_insert
parts_over_threshold = parts_count_in_partition - parts_to_delay_insert + 1
delay_milliseconds = max(min_delay_to_insert_ms, (max_delay_to_insert * 1000) * parts_over_threshold / allowed_parts_over_threshold)
For example, if a partition has 224 active parts and parts_to_throw_insert = 300, parts_to_delay_insert = 150, max_delay_to_insert = 1, min_delay_to_insert_ms = 10, INSERT
is delayed for max( 10, 1 * 1000 * (224 - 150 + 1) / (300 - 150) ) = 500
milliseconds.
max_parts_in_total
If the total number of active parts in all partitions of a table exceeds the max_parts_in_total
value INSERT
is interrupted with the Too many parts (N)
exception.
Possible values:
- Any positive integer.
Default value: 100000.
A large number of parts in a table reduces performance of ClickHouse queries and increases ClickHouse boot time. Most often this is a consequence of an incorrect design (mistakes when choosing a partitioning strategy - too small partitions).
simultaneous_parts_removal_limit
If there are a lot of outdated parts cleanup thread will try to delete up to simultaneous_parts_removal_limit
parts during one iteration.
simultaneous_parts_removal_limit
set to 0
means unlimited.
Default value: 0.
replicated_deduplication_window
The number of most recently inserted blocks for which ClickHouse Keeper stores hash sums to check for duplicates.
Possible values:
- Any positive integer.
- 0 (disable deduplication)
Default value: 1000.
The Insert
command creates one or more blocks (parts). For insert deduplication, when writing into replicated tables, ClickHouse writes the hash sums of the created parts into ClickHouse Keeper. Hash sums are stored only for the most recent replicated_deduplication_window
blocks. The oldest hash sums are removed from ClickHouse Keeper.
A large number of replicated_deduplication_window
slows down Inserts
because it needs to compare more entries.
The hash sum is calculated from the composition of the field names and types and the data of the inserted part (stream of bytes).
non_replicated_deduplication_window
The number of the most recently inserted blocks in the non-replicated MergeTree table for which hash sums are stored to check for duplicates.
Possible values:
- Any positive integer.
- 0 (disable deduplication).
Default value: 0.
A deduplication mechanism is used, similar to replicated tables (see replicated_deduplication_window setting). The hash sums of the created parts are written to a local file on a disk.
replicated_deduplication_window_seconds
The number of seconds after which the hash sums of the inserted blocks are removed from ClickHouse Keeper.
Possible values:
- Any positive integer.
Default value: 604800 (1 week).
Similar to replicated_deduplication_window, replicated_deduplication_window_seconds
specifies how long to store hash sums of blocks for insert deduplication. Hash sums older than replicated_deduplication_window_seconds
are removed from ClickHouse Keeper, even if they are less than replicated_deduplication_window
.
The time is relative to the time of the most recent record, not to the wall time. If it's the only record it will be stored forever.
replicated_deduplication_window_for_async_inserts
The number of most recently async inserted blocks for which ClickHouse Keeper stores hash sums to check for duplicates.
Possible values:
- Any positive integer.
- 0 (disable deduplication for async_inserts)
Default value: 10000.
The Async Insert command will be cached in one or more blocks (parts). For insert deduplication, when writing into replicated tables, ClickHouse writes the hash sums of each insert into ClickHouse Keeper. Hash sums are stored only for the most recent replicated_deduplication_window_for_async_inserts
blocks. The oldest hash sums are removed from ClickHouse Keeper.
A large number of replicated_deduplication_window_for_async_inserts
slows down Async Inserts
because it needs to compare more entries.
The hash sum is calculated from the composition of the field names and types and the data of the insert (stream of bytes).
replicated_deduplication_window_seconds_for_async_inserts
The number of seconds after which the hash sums of the async inserts are removed from ClickHouse Keeper.
Possible values:
- Any positive integer.
Default value: 604800 (1 week).
Similar to replicated_deduplication_window_for_async_inserts, replicated_deduplication_window_seconds_for_async_inserts
specifies how long to store hash sums of blocks for async insert deduplication. Hash sums older than replicated_deduplication_window_seconds_for_async_inserts
are removed from ClickHouse Keeper, even if they are less than replicated_deduplication_window_for_async_inserts
.
The time is relative to the time of the most recent record, not to the wall time. If it's the only record it will be stored forever.
use_async_block_ids_cache
If true, we cache the hash sums of the async inserts.
Possible values:
- true, false
Default value: false.
A block bearing multiple async inserts will generate multiple hash sums. When some of the inserts are duplicated, keeper will only return one duplicated hash sum in one RPC, which will cause unnecessary RPC retries. This cache will watch the hash sums path in Keeper. If updates are watched in the Keeper, the cache will update as soon as possible, so that we are able to filter the duplicated inserts in the memory.
async_block_ids_cache_min_update_interval_ms
The minimum interval (in milliseconds) to update the use_async_block_ids_cache
Possible values:
- Any positive integer.
Default value: 100.
Normally, the use_async_block_ids_cache
updates as soon as there are updates in the watching keeper path. However, the cache updates might be too frequent and become a heavy burden. This minimum interval prevents the cache from updating too fast. Note that if we set this value too long, the block with duplicated inserts will have a longer retry time.
max_replicated_logs_to_keep
How many records may be in the ClickHouse Keeper log if there is inactive replica. An inactive replica becomes lost when when this number exceed.
Possible values:
- Any positive integer.
Default value: 1000
min_replicated_logs_to_keep
Keep about this number of last records in ZooKeeper log, even if they are obsolete. It doesn't affect work of tables: used only to diagnose ZooKeeper log before cleaning.
Possible values:
- Any positive integer.
Default value: 10
prefer_fetch_merged_part_time_threshold
If the time passed since a replication log (ClickHouse Keeper or ZooKeeper) entry creation exceeds this threshold, and the sum of the size of parts is greater than prefer_fetch_merged_part_size_threshold
, then prefer fetching merged part from a replica instead of doing merge locally. This is to speed up very long merges.
Possible values:
- Any positive integer.
Default value: 3600
prefer_fetch_merged_part_size_threshold
If the sum of the size of parts exceeds this threshold and the time since a replication log entry creation is greater than prefer_fetch_merged_part_time_threshold
, then prefer fetching merged part from a replica instead of doing merge locally. This is to speed up very long merges.
Possible values:
- Any positive integer.
Default value: 10,737,418,240
execute_merges_on_single_replica_time_threshold
When this setting has a value greater than zero, only a single replica starts the merge immediately, and other replicas wait up to that amount of time to download the result instead of doing merges locally. If the chosen replica doesn't finish the merge during that amount of time, fallback to standard behavior happens.
Possible values:
- Any positive integer.
Default value: 0 (seconds)
remote_fs_execute_merges_on_single_replica_time_threshold
When this setting has a value greater than zero only a single replica starts the merge immediately if merged part on shared storage and allow_remote_fs_zero_copy_replication
is enabled.
Zero-copy replication is disabled by default in ClickHouse version 22.8 and higher. This feature is not recommended for production use.
Possible values:
- Any positive integer.
Default value: 10800
try_fetch_recompressed_part_timeout
Timeout (in seconds) before starting merge with recompression. During this time ClickHouse tries to fetch recompressed part from replica which assigned this merge with recompression.
Recompression works slow in most cases, so we don't start merge with recompression until this timeout and trying to fetch recompressed part from replica which assigned this merge with recompression.
Possible values:
- Any positive integer.
Default value: 7200
always_fetch_merged_part
If true, this replica never merges parts and always downloads merged parts from other replicas.
Possible values:
- true, false
Default value: false
max_suspicious_broken_parts
Max broken parts, if more - deny automatic deletion.
Possible values:
- Any positive integer.
Default value: 100
max_suspicious_broken_parts_bytes
Max size of all broken parts, if more - deny automatic deletion.
Possible values:
- Any positive integer.
Default value: 1,073,741,824
max_files_to_modify_in_alter_columns
Do not apply ALTER if number of files for modification(deletion, addition) is greater than this setting.
Possible values:
- Any positive integer.
Default value: 75
max_files_to_remove_in_alter_columns
Do not apply ALTER, if the number of files for deletion is greater than this setting.
Possible values:
- Any positive integer.
Default value: 50
replicated_max_ratio_of_wrong_parts
If the ratio of wrong parts to total number of parts is less than this - allow to start.
Possible values:
- Float, 0.0 - 1.0
Default value: 0.5
replicated_max_parallel_fetches_for_host
Limit parallel fetches from endpoint (actually pool size).
Possible values:
- Any positive integer.
Default value: 15
replicated_fetches_http_connection_timeout
HTTP connection timeout for part fetch requests. Inherited from default profile http_connection_timeout
if not set explicitly.
Possible values:
- Any positive integer.
Default value: Inherited from default profile http_connection_timeout
if not set explicitly.
replicated_can_become_leader
If true, replicated tables replicas on this node will try to acquire leadership.
Possible values:
- true, false
Default value: true
zookeeper_session_expiration_check_period
ZooKeeper session expiration check period, in seconds.
Possible values:
- Any positive integer.
Default value: 60
detach_old_local_parts_when_cloning_replica
Do not remove old local parts when repairing lost replica.
Possible values:
- true, false
Default value: true
replicated_fetches_http_connection_timeout
HTTP connection timeout (in seconds) for part fetch requests. Inherited from default profile http_connection_timeout if not set explicitly.
Possible values:
- Any positive integer.
- 0 - Use value of
http_connection_timeout
.
Default value: 0.
replicated_fetches_http_send_timeout
HTTP send timeout (in seconds) for part fetch requests. Inherited from default profile http_send_timeout if not set explicitly.
Possible values:
- Any positive integer.
- 0 - Use value of
http_send_timeout
.
Default value: 0.
replicated_fetches_http_receive_timeout
HTTP receive timeout (in seconds) for fetch part requests. Inherited from default profile http_receive_timeout if not set explicitly.
Possible values:
- Any positive integer.
- 0 - Use value of
http_receive_timeout
.
Default value: 0.
max_replicated_fetches_network_bandwidth
Limits the maximum speed of data exchange over the network in bytes per second for replicated fetches. This setting is applied to a particular table, unlike the max_replicated_fetches_network_bandwidth_for_server setting, which is applied to the server.
You can limit both server network and network for a particular table, but for this the value of the table-level setting should be less than server-level one. Otherwise the server considers only the max_replicated_fetches_network_bandwidth_for_server
setting.
The setting isn't followed perfectly accurately.
Possible values:
- Positive integer.
- 0 — Unlimited.
Default value: 0
.
Usage
Could be used for throttling speed when replicating data to add or replace new nodes.
max_replicated_sends_network_bandwidth
Limits the maximum speed of data exchange over the network in bytes per second for replicated sends. This setting is applied to a particular table, unlike the max_replicated_sends_network_bandwidth_for_server setting, which is applied to the server.
You can limit both server network and network for a particular table, but for this the value of the table-level setting should be less than server-level one. Otherwise the server considers only the max_replicated_sends_network_bandwidth_for_server
setting.
The setting isn't followed perfectly accurately.
Possible values:
- Positive integer.
- 0 — Unlimited.
Default value: 0
.
Usage
Could be used for throttling speed when replicating data to add or replace new nodes.
old_parts_lifetime
The time (in seconds) of storing inactive parts to protect against data loss during spontaneous server reboots.
Possible values:
- Any positive integer.
Default value: 480.
After merging several parts into a new part, ClickHouse marks the original parts as inactive and deletes them only after old_parts_lifetime
seconds.
Inactive parts are removed if they are not used by current queries, i.e. if the refcount
of the part is 1.
fsync
is not called for new parts, so for some time new parts exist only in the server's RAM (OS cache). If the server is rebooted spontaneously, new parts can be lost or damaged.
To protect data inactive parts are not deleted immediately.
During startup ClickHouse checks the integrity of the parts.
If the merged part is damaged ClickHouse returns the inactive parts to the active list, and later merges them again. Then the damaged part is renamed (the broken_
prefix is added) and moved to the detached
folder.
If the merged part is not damaged, then the original inactive parts are renamed (the ignored_
prefix is added) and moved to the detached
folder.
The default dirty_expire_centisecs
value (a Linux kernel setting) is 30 seconds (the maximum time that written data is stored only in RAM), but under heavy loads on the disk system data can be written much later. Experimentally, a value of 480 seconds was chosen for old_parts_lifetime
, during which a new part is guaranteed to be written to disk.
max_bytes_to_merge_at_max_space_in_pool
The maximum total parts size (in bytes) to be merged into one part, if there are enough resources available.
max_bytes_to_merge_at_max_space_in_pool
-- roughly corresponds to the maximum possible part size created by an automatic background merge.
Possible values:
- Any positive integer.
Default value: 161061273600 (150 GB).
The merge scheduler periodically analyzes the sizes and number of parts in partitions, and if there is enough free resources in the pool, it starts background merges. Merges occur until the total size of the source parts is larger than max_bytes_to_merge_at_max_space_in_pool
.
Merges initiated by OPTIMIZE FINAL ignore max_bytes_to_merge_at_max_space_in_pool
and merge parts only taking into account available resources (free disk's space) until one part remains in the partition.
max_bytes_to_merge_at_min_space_in_pool
The maximum total part size (in bytes) to be merged into one part, with the minimum available resources in the background pool.
Possible values:
- Any positive integer.
Default value: 1048576 (1 MB)
max_bytes_to_merge_at_min_space_in_pool
defines the maximum total size of parts which can be merged despite the lack of available disk space (in pool). This is necessary to reduce the number of small parts and the chance of Too many parts
errors.
Merges book disk space by doubling the total merged parts sizes. Thus, with a small amount of free disk space, a situation may happen that there is free space, but this space is already booked by ongoing large merges, so other merges unable to start, and the number of small parts grows with every insert.
merge_max_block_size
The number of rows that are read from the merged parts into memory.
Possible values:
- Any positive integer.
Default value: 8192
Merge reads rows from parts in blocks of merge_max_block_size
rows, then merges and writes the result into a new part. The read block is placed in RAM, so merge_max_block_size
affects the size of the RAM required for the merge. Thus, merges can consume a large amount of RAM for tables with very wide rows (if the average row size is 100kb, then when merging 10 parts, (100kb 10 8192) = ~ 8GB of RAM). By decreasing merge_max_block_size
, you can reduce the amount of RAM required for a merge but slow down a merge.
number_of_free_entries_in_pool_to_lower_max_size_of_merge
When there is less than specified number of free entries in pool (or replicated queue), start to lower maximum size of merge to process (or to put in queue). This is to allow small merges to process - not filling the pool with long running merges.
Possible values:
- Any positive integer.
Default value: 8
number_of_free_entries_in_pool_to_execute_mutation
When there is less than specified number of free entries in pool, do not execute part mutations. This is to leave free threads for regular merges and avoid "Too many parts".
Possible values:
- Any positive integer.
Default value: 20
Usage
The value of the number_of_free_entries_in_pool_to_execute_mutation
setting should be less than the value of the background_pool_size * background_merges_mutations_concurrency_ratio. Otherwise, ClickHouse throws an exception.
max_part_loading_threads
The maximum number of threads that read parts when ClickHouse starts.
Possible values:
- Any positive integer.
Default value: auto (number of CPU cores).
During startup ClickHouse reads all parts of all tables (reads files with metadata of parts) to build a list of all parts in memory. In some systems with a large number of parts this process can take a long time, and this time might be shortened by increasing max_part_loading_threads
(if this process is not CPU and disk I/O bound).
max_partitions_to_read
Limits the maximum number of partitions that can be accessed in one query.
The setting value specified when the table is created can be overridden via query-level setting.
Possible values:
- Any positive integer.
Default value: -1 (unlimited).
You can also specify a query complexity setting max_partitions_to_read at a query / session / profile level.
min_age_to_force_merge_seconds
Merge parts if every part in the range is older than the value of min_age_to_force_merge_seconds
.
Possible values:
- Positive integer.
Default value: 0 — Disabled.
min_age_to_force_merge_on_partition_only
Whether min_age_to_force_merge_seconds
should be applied only on the entire partition and not on subset.
Possible values:
- true, false
Default value: false
number_of_free_entries_in_pool_to_execute_optimize_entire_partition
When there is less than specified number of free entries in pool, do not execute optimizing entire partition in the background (this task generated when set min_age_to_force_merge_seconds
and enable min_age_to_force_merge_on_partition_only
). This is to leave free threads for regular merges and avoid "Too many parts".
Possible values:
- Positive integer.
Default value: 25
The value of the number_of_free_entries_in_pool_to_execute_optimize_entire_partition
setting should be less than the value of the background_pool_size * background_merges_mutations_concurrency_ratio. Otherwise, ClickHouse throws an exception.
allow_floating_point_partition_key
Enables to allow floating-point number as a partition key.
Possible values:
- 0 — Floating-point partition key not allowed.
- 1 — Floating-point partition key allowed.
Default value: 0
.
check_sample_column_is_correct
Enables the check at table creation, that the data type of a column for sampling or sampling expression is correct. The data type must be one of unsigned integer types: UInt8
, UInt16
, UInt32
, UInt64
.
Possible values:
- true — The check is enabled.
- false — The check is disabled at table creation.
Default value: true
.
By default, the ClickHouse server checks at table creation the data type of a column for sampling or sampling expression. If you already have tables with incorrect sampling expression and do not want the server to raise an exception during startup, set check_sample_column_is_correct
to false
.
min_bytes_to_rebalance_partition_over_jbod
Sets minimal amount of bytes to enable balancing when distributing new big parts over volume disks JBOD.
Possible values:
- Positive integer.
- 0 — Balancing is disabled.
Default value: 0
.
Usage
The value of the min_bytes_to_rebalance_partition_over_jbod
setting should not be less than the value of the max_bytes_to_merge_at_max_space_in_pool / 1024. Otherwise, ClickHouse throws an exception.
detach_not_byte_identical_parts
Enables or disables detaching a data part on a replica after a merge or a mutation, if it is not byte-identical to data parts on other replicas. If disabled, the data part is removed. Activate this setting if you want to analyze such parts later.
The setting is applicable to MergeTree
tables with enabled data replication.
Possible values:
- 0 — Parts are removed.
- 1 — Parts are detached.
Default value: 0
.
merge_tree_clear_old_temporary_directories_interval_seconds
Sets the interval in seconds for ClickHouse to execute the cleanup of old temporary directories.
Possible values:
- Any positive integer.
Default value: 60
seconds.
merge_tree_clear_old_parts_interval_seconds
Sets the interval in seconds for ClickHouse to execute the cleanup of old parts, WALs, and mutations.
Possible values:
- Any positive integer.
Default value: 1
second.
max_concurrent_queries
Max number of concurrently executed queries related to the MergeTree table. Queries will still be limited by other max_concurrent_queries
settings.
Possible values:
- Positive integer.
- 0 — No limit.
Default value: 0
(no limit).
Example
<max_concurrent_queries>50</max_concurrent_queries>
min_marks_to_honor_max_concurrent_queries
The minimal number of marks read by the query for applying the max_concurrent_queries setting. Note that queries will still be limited by other max_concurrent_queries
settings.
Possible values:
- Positive integer.
- 0 — Disabled (
max_concurrent_queries
limit applied to no queries).
Default value: 0
(limit never applied).
Example
<min_marks_to_honor_max_concurrent_queries>10</min_marks_to_honor_max_concurrent_queries>
ratio_of_defaults_for_sparse_serialization
Minimal ratio of the number of default values to the number of all values in a column. Setting this value causes the column to be stored using sparse serializations.
If a column is sparse (contains mostly zeros), ClickHouse can encode it in a sparse format and automatically optimize calculations - the data does not require full decompression during queries. To enable this sparse serialization, define the ratio_of_defaults_for_sparse_serialization
setting to be less than 1.0. If the value is greater than or equal to 1.0, then the columns will be always written using the normal full serialization.
Possible values:
- Float between 0 and 1 to enable sparse serialization
- 1.0 (or greater) if you do not want to use sparse serialization
Default value: 0.9375
Example
Notice the s
column in the following table is an empty string for 95% of the rows. In my_regular_table
we do not use sparse serialization, and in my_sparse_table
we set ratio_of_defaults_for_sparse_serialization
to 0.95:
CREATE TABLE my_regular_table
(
`id` UInt64,
`s` String
)
ENGINE = MergeTree
ORDER BY id;
INSERT INTO my_regular_table
SELECT
number AS id,
number % 20 = 0 ? toString(number): '' AS s
FROM
numbers(10000000);
CREATE TABLE my_sparse_table
(
`id` UInt64,
`s` String
)
ENGINE = MergeTree
ORDER BY id
SETTINGS ratio_of_defaults_for_sparse_serialization = 0.95;
INSERT INTO my_sparse_table
SELECT
number,
number % 20 = 0 ? toString(number): ''
FROM
numbers(10000000);
Notice the s
column in my_sparse_table
uses less storage space on disk:
SELECT table, name, data_compressed_bytes, data_uncompressed_bytes FROM system.columns
WHERE table LIKE 'my_%_table';
┌─table────────────┬─name─┬─data_compressed_bytes─┬─data_uncompressed_bytes─┐
│ my_regular_table │ id │ 37790741 │ 75488328 │
│ my_regular_table │ s │ 2451377 │ 12683106 │
│ my_sparse_table │ id │ 37790741 │ 75488328 │
│ my_sparse_table │ s │ 2283454 │ 9855751 │
└──────────────────┴──────┴───────────────────────┴─────────────────────────┘
You can verify if a column is using the sparse encoding by viewing the serialization_kind
column of the system.parts_columns
table:
SELECT column, serialization_kind FROM system.parts_columns
WHERE table LIKE 'my_sparse_table';
You can see which parts of s
were stored using the sparse serialization:
┌─column─┬─serialization_kind─┐
│ id │ Default │
│ s │ Default │
│ id │ Default │
│ s │ Default │
│ id │ Default │
│ s │ Sparse │
│ id │ Default │
│ s │ Sparse │
│ id │ Default │
│ s │ Sparse │
│ id │ Default │
│ s │ Sparse │
│ id │ Default │
│ s │ Sparse │
│ id │ Default │
│ s │ Sparse │
│ id │ Default │
│ s │ Sparse │
│ id │ Default │
│ s │ Sparse │
│ id │ Default │
│ s │ Sparse │
└────────┴────────────────────┘
replace_long_file_name_to_hash
If the file name for column is too long (more than max_file_name_length
bytes) replace it to SipHash128. Default value: false
.
max_file_name_length
The maximal length of the file name to keep it as is without hashing. Takes effect only if setting replace_long_file_name_to_hash
is enabled. The value of this setting does not include the length of file extension. So, it is recommended to set it below the maximum filename length (usually 255 bytes) with some gap to avoid filesystem errors. Default value: 127.
allow_experimental_block_number_column
Persists virtual column _block_number
on merges.
Default value: false.
exclude_deleted_rows_for_part_size_in_merge
If enabled, estimated actual size of data parts (i.e., excluding those rows that have been deleted through DELETE FROM
) will be used when selecting parts to merge. Note that this behavior is only triggered for data parts affected by DELETE FROM
executed after this setting is enabled.
Possible values:
- true, false
Default value: false
See Also
load_existing_rows_count_for_old_parts
If enabled along with exclude_deleted_rows_for_part_size_in_merge, deleted rows count for existing data parts will be calculated during table starting up. Note that it may slow down start up table loading.
Possible values:
- true, false
Default value: false
See Also
use_compact_variant_discriminators_serialization
Enables compact mode for binary serialization of discriminators in Variant data type. This mode allows to use significantly less memory for storing discriminators in parts when there is mostly one variant or a lot of NULL values.
Default value: true
merge_workload
Used to regulate how resources are utilized and shared between merges and other workloads. Specified value is used as workload
setting value for background merges of this table. If not specified (empty string), then server setting merge_workload
is used instead.
Default value: an empty string
See Also
mutation_workload
Used to regulate how resources are utilized and shared between mutations and other workloads. Specified value is used as workload
setting value for background mutations of this table. If not specified (empty string), then server setting mutation_workload
is used instead.
Default value: an empty string
See Also
optimize_row_order
Controls if the row order should be optimized during inserts to improve the compressability of the newly inserted table part.
Only has an effect for ordinary MergeTree-engine tables. Does nothing for specialized MergeTree engine tables (e.g. CollapsingMergeTree).
MergeTree tables are (optionally) compressed using compression codecs. Generic compression codecs such as LZ4 and ZSTD achieve maximum compression rates if the data exposes patterns. Long runs of the same value typically compress very well.
If this setting is enabled, ClickHouse attempts to store the data in newly inserted parts in a row order that minimizes the number of equal-value runs across the columns of the new table part. In other words, a small number of equal-value runs mean that individual runs are long and compress well.
Finding the optimal row order is computationally infeasible (NP hard). Therefore, ClickHouse uses a heuristics to quickly find a row order which still improves compression rates over the original row order.
Heuristics for finding a row order
It is generally possible to shuffle the rows of a table (or table part) freely as SQL considers the same table (table part) in different row order equivalent.
This freedom of shuffling rows is restricted when a primary key is defined for the table.
In ClickHouse, a primary key C1, C2, ..., CN
enforces that the table rows are sorted by columns C1
, C2
, ... Cn
(clustered index).
As a result, rows can only be shuffled within "equivalence classes" of row, i.e. rows which have the same values in their primary key columns.
The intuition is that primary keys with high-cardinality, e.g. primary keys involving a DateTime64
timestamp column, lead to many small equivalence classes.
Likewise, tables with a low-cardinality primary key, create few and large equivalence classes.
A table with no primary key represents the extreme case of a single equivalence class which spans all rows.
The fewer and the larger the equivalence classes are, the higher the degree of freedom when re-shuffling rows.
The heuristics applied to find the best row order within each equivalence class is suggested by D. Lemire, O. Kaser in Reordering columns for smaller indexes and based on sorting the rows within each equivalence class by ascending cardinality of the non-primary key columns. It performs three steps:
- Find all equivalence classes based on the row values in primary key columns.
- For each equivalence class, calculate (usually estimate) the cardinalities of the non-primary-key columns.
- For each equivalence class, sort the rows in order of ascending non-primary-key column cardinality.
If enabled, insert operations incur additional CPU costs to analyze and optimize the row order of the new data. INSERTs are expected to take 30-50% longer depending on the data characteristics. Compression rates of LZ4 or ZSTD improve on average by 20-40%.
This setting works best for tables with no primary key or a low-cardinality primary key, i.e. a table with only few distinct primary key values.
High-cardinality primary keys, e.g. involving timestamp columns of type DateTime64
, are not expected to benefit from this setting.
lightweight_mutation_projection_mode
By default, lightweight delete DELETE
does not work for tables with projections. This is because rows in a projection may be affected by a DELETE
operation. So the default value would be throw
.
However, this option can change the behavior. With the value either drop
or rebuild
, deletes will work with projections. drop
would delete the projection so it might be fast in the current query as projection gets deleted but slow in future queries as no projection attached.
rebuild
would rebuild the projection which might affect the performance of the current query, but might speedup for future queries. A good thing is that these options would only work in the part level,
which means projections in the part that don't get touched would stay intact instead of triggering any action like drop or rebuild.
Possible values:
- throw, drop, rebuild
Default value: throw
deduplicate_merge_projection_mode
Whether to allow create projection for the table with non-classic MergeTree, that is not (Replicated, Shared) MergeTree. If allowed, what is the action when merge projections, either drop or rebuild. So classic MergeTree would ignore this setting.
It also controls OPTIMIZE DEDUPLICATE
as well, but has effect on all MergeTree family members. Similar to the option lightweight_mutation_projection_mode
, it is also part level.
Possible values:
- throw, drop, rebuild
Default value: throw