There are various configuration parameters that affect the behaviour of Citus. These include both standard PostgreSQL parameters and Citus specific parameters. To learn more about PostgreSQL configuration parameters, you can visit the run time configuration section of PostgreSQL documentation.
The rest of this reference aims at discussing Citus specific configuration parameters. These parameters can be set similar to PostgreSQL parameters by modifying postgresql.conf or by using the SET command.
As an example you can update a setting with:
ALTER DATABASE citus SET citus.multi_task_query_log_level = 'log';
Citus tracks worker nodes’ locations and their membership in a shared hash table on the coordinator node. This configuration value limits the size of the hash table, and consequently the number of worker nodes that can be tracked. The default for this setting is 2048. This parameter can only be set at server start and is effective on the coordinator node.
Sets the policy to use when choosing nodes for SELECT queries. If this is set to ‘always’, then the planner will query only nodes which are marked as ‘secondary’ noderole in pg_dist_node.
The supported values for this enum are:
- never: (default) All reads happen on primary nodes.
- always: Reads run against secondary nodes instead, and insert/update statements are disabled.
Informs the coordinator node planner which cluster it coordinates. Once cluster_name is set, the planner will query worker nodes in that cluster alone.
Upgrading Citus version requires a server restart (to pick up the new shared-library), as well as running an ALTER EXTENSION UPDATE command. The failure to execute both steps could potentially cause errors or crashes. Citus thus validates the version of the code and that of the extension match, and errors out if they don’t.
This value defaults to true, and is effective on the coordinator. In rare cases, complex upgrade processes may require setting this parameter to false, thus disabling the check.
Whether to log distributed deadlock detection related processing in the server log. It defaults to false.
citus.distributed_deadlock_detection_factor (floating point)¶
Sets the time to wait before checking for distributed deadlocks. In particular the time to wait will be this value multiplied by PostgreSQL’s deadlock_timeout setting. The default value is
2. A value of
-1 disables distributed deadlock detection.
citus.node_connection_timeout GUC sets the maximum duration (in milliseconds) to wait for connection establishment. Citus raises an error if the timeout elapses before at least one worker connection is established. This GUC affects connections from the coordinator to workers, and workers to each other.
- Default: thirty seconds
- Minimum: ten milliseconds
- Maximum: one hour
-- set to 60 seconds ALTER DATABASE foo SET citus.node_connection_timeout = 60000;
citus.node_conninfo GUC sets non-sensitive libpq connection parameters used for all inter-node connections.
-- key=value pairs separated by spaces. -- For example, ssl options: ALTER DATABASE foo SET citus.node_conninfo = 'sslrootcert=/path/to/citus.crt sslmode=verify-full';
Citus honors only a whitelisted subset of the options, namely:
- sslmode (defaults to “require” as of Citus 8.1)
(† = subject to the runtime presence of optional PostgreSQL features)
node_conninfo setting takes effect only on newly opened connections. To force all connections to use the new settings, make sure to reload the postgres configuration:
Citus versions prior to 9.2.4 require a full database restart to force all connections to use the new setting.
This GUC is a part of Citus Enterprise. Please contact us to obtain this functionality.
Sets the frequency at which the maintenance daemon removes records from citus_stat_statements that are unmatched in
pg_stat_statements. This configuration value sets the time interval between purges in seconds, with a default value of 10. A value of 0 disables the purges.
SET citus.stat_statements_purge_interval TO 5;
This parameter is effective on the coordinator and can be changed at runtime.
This GUC is a part of Citus Enterprise. Please contact us to obtain this functionality.
The maximum number of rows to store in citus_stat_statements. Defaults to 50000, and may be changed to any value in the range 1000 - 10000000. Note that each row requires 140 bytes of storage, so setting stat_statements_max to its maximum value of 10M would consume 1.4GB of memory.
Changing this GUC will not take effect until PostgreSQL is restarted.
Sets the commit protocol to use when performing COPY on a hash distributed table. On each individual shard placement, the COPY is performed in a transaction block to ensure that no data is ingested if an error occurs during the COPY. However, there is a particular failure case in which the COPY succeeds on all placements, but a (hardware) failure occurs before all transactions commit. This parameter can be used to prevent data loss in that case by choosing between the following commit protocols:
- 2pc: (default) The transactions in which COPY is performed on the shard placements are first prepared using PostgreSQL’s two-phase commit and then committed. Failed commits can be manually recovered or aborted using COMMIT PREPARED or ROLLBACK PREPARED, respectively. When using 2pc, max_prepared_transactions should be increased on all the workers, typically to the same value as max_connections.
- 1pc: The transactions in which COPY is performed on the shard placements are committed in a single round. Data may be lost if a commit fails after COPY succeeds on all placements (rare).
Sets the replication factor for shards i.e. the number of nodes on which shards will be placed and defaults to 1. This parameter can be set at run-time and is effective on the coordinator. The ideal value for this parameter depends on the size of the cluster and rate of node failure. For example, you may want to increase this replication factor if you run large clusters and observe node failures on a more frequent basis.
Sets the shard count for hash-partitioned tables and defaults to 32. This value is used by the create_distributed_table UDF when creating hash-partitioned tables. This parameter can be set at run-time and is effective on the coordinator.
Sets the maximum size to which a shard will grow before it gets split and defaults to 1GB. When the source file’s size (which is used for staging) for one shard exceeds this configuration value, the database ensures that a new shard gets created. This parameter can be set at run-time and is effective on the coordinator.
Reference table shards must be placed on all nodes which have distributed tables. By default, reference table shards are copied to a node at node activation time, that is, when such functions as master_add_node or master_activate_node are called. However node activation might be an inconvenient time to copy the placements, because it can take a long time when there are large reference tables.
You can defer reference table replication by setting the
citus.replicate_reference_tables_on_activate GUC to ‘off’. Reference table
replication will then happen when we create new shards on the node. For instance,
when calling create_distributed_table, create_reference_table,
upgrade_to_reference_table, or when the shard rebalancer moves shards to
the new node.
The default value for this GUC is ‘on’.
Sets the number of rows to fetch per task for limit clause optimization. In some cases, select queries with limit clauses may need to fetch all rows from each task to generate results. In those cases, and where an approximation would produce meaningful results, this configuration value sets the number of rows to fetch from each shard. Limit approximations are disabled by default and this parameter is set to -1. This value can be set at run-time and is effective on the coordinator.
citus.count_distinct_error_rate (floating point)¶
Citus can calculate count(distinct) approximates using the postgresql-hll extension. This configuration entry sets the desired error rate when calculating count(distinct). 0.0, which is the default, disables approximations for count(distinct); and 1.0 provides no guarantees about the accuracy of results. We recommend setting this parameter to 0.005 for best results. This value can be set at run-time and is effective on the coordinator.
Sets the policy to use when assigning tasks to workers. The coordinator assigns tasks to workers based on shard locations. This configuration value specifies the policy to use when making these assignments. Currently, there are three possible task assignment policies which can be used.
- greedy: The greedy policy is the default and aims to evenly distribute tasks across workers.
- round-robin: The round-robin policy assigns tasks to workers in a round-robin fashion alternating between different replicas. This enables much better cluster utilization when the shard count for a table is low compared to the number of workers.
- first-replica: The first-replica policy assigns tasks on the basis of the insertion order of placements (replicas) for the shards. In other words, the fragment query for a shard is simply assigned to the worker which has the first replica of that shard. This method allows you to have strong guarantees about which shards will be used on which nodes (i.e. stronger memory residency guarantees).
This parameter can be set at run-time and is effective on the coordinator.
Intermediate Data Transfer¶
Use the binary copy format to transfer intermediate data between workers. During large table joins, Citus may have to dynamically repartition and shuffle data between different workers. By default, this data is transferred in text format. Enabling this parameter instructs the database to use PostgreSQL’s binary serialization format to transfer this data. This parameter is effective on the workers and needs to be changed in the postgresql.conf file. After editing the config file, users can send a SIGHUP signal or restart the server for this change to take effect.
The maximum size in KB of intermediate results for CTEs that are unable to be pushed down to worker nodes for execution, and for complex subqueries. The default is 1GB, and a value of -1 means no limit. Queries exceeding the limit will be canceled and produce an error message.
Specifies whether to automatically propagate DDL changes from the coordinator to all workers. The default value is true. Because some schema changes require an access exclusive lock on tables and because the automatic propagation applies to all workers sequentially it can make a Citus cluster temporarily less responsive. You may choose to disable this setting and propagate changes manually.
For a list of DDL propagation support, see Modifying Tables.
Citus enforces commutativity rules and acquires appropriate locks for modify operations in order to guarantee correctness of behavior. For example, it assumes that an INSERT statement commutes with another INSERT statement, but not with an UPDATE or DELETE statement. Similarly, it assumes that an UPDATE or DELETE statement does not commute with another UPDATE or DELETE statement. This means that UPDATEs and DELETEs require Citus to acquire stronger locks.
If you have UPDATE statements that are commutative with your INSERTs or other UPDATEs, then you can relax these commutativity assumptions by setting this parameter to true. When this parameter is set to true, all commands are considered commutative and claim a shared lock, which can improve overall throughput. This parameter can be set at runtime and is effective on the coordinator.
Sets a log-level for any query which generates more than one task (i.e. which hits more than one shard). This is useful during a multi-tenant application migration, as you can choose to error or warn for such queries, to find them and add a tenant_id filter to them. This parameter can be set at runtime and is effective on the coordinator. The default value for this parameter is ‘off’.
The supported values for this enum are:
- off: Turn off logging any queries which generate multiple tasks (i.e. span multiple shards)
- debug: Logs statement at DEBUG severity level.
- log: Logs statement at LOG severity level. The log line will include the SQL query that was run.
- notice: Logs statement at NOTICE severity level.
- warning: Logs statement at WARNING severity level.
- error: Logs statement at ERROR severity level.
Note that it may be useful to use
error during development testing, and a lower log-level like
log during actual production deployment. Choosing
log will cause multi-task queries to appear in the database logs with the query itself shown after “STATEMENT.”
LOG: multi-task query about to be executed HINT: Queries are split to multiple tasks if they have to be split into several queries on the workers. STATEMENT: select * from foo;
Determines which SET commands are propagated from the coordinator to workers. The default value for this parameter is ‘none’.
The supported values are:
- none: no SET commands are propagated.
- local: only SET LOCAL commands are propagated.
Ordinarily, attempting to perform Repartition joins with the adaptive executor will fail with an error message. However setting
citus.enable_repartition_joins to true allows Citus to perform the join. The default value is false.
By default, an INSERT INTO … SELECT statement that cannot be pushed down will attempt to repartition rows from the SELECT statement and transfer them between workers for insertion. However, if the target table has too many shards then repartitioning will probably not perform well. The overhead of processing the shard intervals when determining how to partition the results is too great. Repartitioning can be disabled manually by setting
citus.enable_repartitioned_insert_select to false.
Setting this parameter to true instructs the coordinator node to use PostgreSQL’s binary serialization format (when applicable) to transfer data with workers. Some column types do not support binary serialization.
Enabling this parameter is mostly useful when the workers must return large
amounts of data. Examples are when a lot of rows are requested, the rows have
many columns, or they use big types such as
hll from the postgresql-hll
The default value is false, which means all results are encoded and transferred in text format.
Whereas citus.max_shared_pool_size (integer) limits worker connections across all
max_adaptive_executor_pool_size limits worker connections from
just the current session. This GUC is useful for:
- Preventing a single backend from getting all the worker resources
- Providing priority management: designate low priority sessions with low max_adaptive_executor_pool_size, and high priority sessions with higher values
The default value is 16.
Time to wait in milliseconds between opening connections to the same worker node.
When the individual tasks of a multi-shard query take very little time, they can often be finished over a single (often already cached) connection. To avoid redundantly opening additional connections, the executor waits between connection attempts for the configured number of milliseconds. At the end of the interval, it increases the number of connections it is allowed to open next time.
For long queries (those taking >500ms), slow start might add latency, but for short queries it’s faster. The default value is 10ms.
Each backend opens connections to the workers to query the shards. At the end of the transaction, the configured number of connections is kept open to speed up subsequent commands. Increasing this value will reduce the latency of multi-shard queries, but will also increase overhead on the workers.
The default value is 1. A larger value such as 2 might be helpful for clusters that use a small number of concurrent sessions, but it’s not wise to go much further (e.g. 16 would be too high).
Simulates the deprecated and now nonexistent real-time executor. This is used to open as many connections as possible to maximize query parallelization.
When this GUC is enabled, Citus will force the adaptive executor to use as many connections as possible while executing a parallel distributed query. If not enabled, the executor might choose to use fewer connections to optimize overall query execution throughput. Internally, setting this true will end up using one connection per task.
One place where this is useful is in a transaction whose first query is lightweight and requires few connections, while a subsequent query would benefit from more connections. Citus decides how many connections to use in a transaction based on the first statement, which can throttle other queries unless we use the GUC to provide a hint.
BEGIN; -- add this hint SET citus.force_max_query_parallelization TO ON; -- a lightweight query that doesn't require many connections SELECT count(*) FROM table WHERE filter = x; -- a query that benefits from more connections, and can obtain -- them since we forced max parallelization above SELECT ... very .. complex .. SQL; COMMIT;
The default value is false.
By default, Citus shows the output of a single, arbitrary task when running EXPLAIN on a distributed query. In most cases, the explain output will be similar across tasks. Occasionally, some of the tasks will be planned differently or have much higher execution times. In those cases, it can be useful to enable this parameter, after which the EXPLAIN output will include all tasks. This may cause the EXPLAIN to take longer.