Citus Utility Functions

This section contains reference information for the User Defined Functions provided by Citus. These functions help in providing additional distributed functionality to Citus other than the standard SQL commands.

Table and Shard DDL

create_distributed_table

The create_distributed_table() function is used to define a distributed table and create its shards if it’s a hash-distributed table. This function takes in a table name, the distribution column and an optional distribution method and inserts appropriate metadata to mark the table as distributed. The function defaults to ‘hash’ distribution if no distribution method is specified. If the table is hash-distributed, the function also creates worker shards based on the shard count and shard replication factor configuration values. If the table contains any rows, they are automatically distributed to worker nodes.

This function replaces usage of master_create_distributed_table() followed by master_create_worker_shards().

Arguments

table_name: Name of the table which needs to be distributed.

distribution_column: The column on which the table is to be distributed.

distribution_type: (Optional) The method according to which the table is to be distributed. Permissible values are append or hash, and defaults to ‘hash’.

colocate_with: (Optional) include current table in the co-location group of another table. By default tables are co-located when they are distributed by columns of the same type, have the same shard count, and have the same replication factor. If you want to break this colocation later, you can use update_distributed_table_colocation. Possible values for colocate_with are default, none to start a new co-location group, or the name of another table to co-locate with that table. (See Co-Locating Tables.)

Keep in mind that the default value of colocate_with does implicit co-location. As Table Co-Location explains, this can be a great thing when tables are related or will be joined. However when two tables are unrelated but happen to use the same datatype for their distribution columns, accidentally co-locating them can decrease performance during shard rebalancing. The table shards will be moved together unnecessarily in a “cascade.” If you want to break this implicit colocation, you can use update_distributed_table_colocation.

If a new distributed table is not related to other tables, it’s best to specify colocate_with => 'none'.

Return Value

N/A

Example

This example informs the database that the github_events table should be distributed by hash on the repo_id column.

SELECT create_distributed_table('github_events', 'repo_id');

-- alternatively, to be more explicit:
SELECT create_distributed_table('github_events', 'repo_id',
                                colocate_with => 'github_repo');

For more examples, see Creating and Modifying Distributed Tables (DDL).

truncate_local_data_after_distributing_table

Truncate all local rows after distributing a table, and prevent constraints from failing due to outdated local records. The truncation cascades to tables having a foreign key to the designated table. If the referring tables are not themselves distributed then truncation is forbidden until they are, to protect referential integrity:

ERROR:  cannot truncate a table referenced in a foreign key constraint by a local table

Truncating local coordinator node table data is safe for distributed tables because their rows, if they have any, are copied to worker nodes during distribution.

Arguments

table_name: Name of the distributed table whose local counterpart on the coordinator node should be truncated.

Return Value

N/A

Example

-- requires that argument is a distributed table
SELECT truncate_local_data_after_distributing_table('public.github_events');

create_reference_table

The create_reference_table() function is used to define a small reference or dimension table. This function takes in a table name, and creates a distributed table with just one shard, replicated to every worker node.

Arguments

table_name: Name of the small dimension or reference table which needs to be distributed.

Return Value

N/A

Example

This example informs the database that the nation table should be defined as a reference table

SELECT create_reference_table('nation');

upgrade_to_reference_table

The upgrade_to_reference_table() function takes an existing distributed table which has a shard count of one, and upgrades it to be a recognized reference table. After calling this function, the table will be as if it had been created with create_reference_table.

Arguments

table_name: Name of the distributed table (having shard count = 1) which will be distributed as a reference table.

Return Value

N/A

Example

This example informs the database that the nation table should be defined as a reference table

SELECT upgrade_to_reference_table('nation');

mark_tables_colocated

The mark_tables_colocated() function takes a distributed table (the source), and a list of others (the targets), and puts the targets into the same co-location group as the source. If the source is not yet in a group, this function creates one, and assigns the source and targets to it.

Usually colocating tables ought to be done at table distribution time via the colocate_with parameter of create_distributed_table. But mark_tables_colocated can take care of it if necessary.

If you want to break colocation of a table, you can use update_distributed_table_colocation.

Arguments

source_table_name: Name of the distributed table whose co-location group the targets will be assigned to match.

target_table_names: Array of names of the distributed target tables, must be non-empty. These distributed tables must match the source table in:

  • distribution method
  • distribution column type
  • replication type
  • shard count

Failing this, Citus will raise an error. For instance, attempting to colocate tables apples and oranges whose distribution column types differ results in:

ERROR:  XX000: cannot colocate tables apples and oranges
DETAIL:  Distribution column types don't match for apples and oranges.

Return Value

N/A

Example

This example puts products and line_items in the same co-location group as stores. The example assumes that these tables are all distributed on a column with matching type, most likely a “store id.”

SELECT mark_tables_colocated('stores', ARRAY['products', 'line_items']);

update_distributed_table_colocation

The update_distributed_table_colocation() function is used to update colocation of a distributed table. This function can also be used to break colocation of a distributed table. Citus will implicitly colocate two tables if the distribution column is the same type, this can be useful if the tables are related and will do some joins. If table A and B are colocated, and table A gets rebalanced, table B will also be rebalanced. If table B does not have a replica identity, the rebalance will fail. Therefore, this function can be useful breaking the implicit colocation in that case.

Both of the arguments should be a hash distributed table, currently we do not support colocation of APPEND or RANGE distributed tables.

Note that this function does not move any data around physically.

Arguments

table_name: Name of the table colocation of which will be updated.

colocate_with: The table to which the table should be colocated with.

If you want to break the colocation of a table, you should specify colocate_with => 'none'.

Return Value

N/A

Example

This example shows that colocation of table A is updated as colocation of table B.

SELECT update_distributed_table_colocation('A', colocate_with => 'B');

Assume that table A and table B are colocated( possibily implicitly), if you want to break the colocation:

SELECT update_distributed_table_colocation('A', colocate_with => 'none');

Now, assume that table A, table B, table C and table D are colocated and you want to colocate table A and table B together, and table C and table D together:

SELECT update_distributed_table_colocation('C', colocate_with => 'none');
SELECT update_distributed_table_colocation('D', colocate_with => 'C');

If you have a hash distributed table named none and you want to update its colocation, you can do:

SELECT update_distributed_table_colocation('"none"', colocate_with => 'some_other_hash_distributed_table');

create_distributed_function

Propagates a function from the coordinator node to workers, and marks it for distributed execution. When a distributed function is called on the coordinator, Citus uses the value of the “distribution argument” to pick a worker node to run the function. Executing the function on workers increases parallelism, and can bring the code closer to data in shards for lower latency.

Note that the Postgres search path is not propagated from the coordinator to workers during distributed function execution, so distributed function code should fully-qualify the names of database objects. Also notices emitted by the functions will not be displayed to the user.

Arguments

function_name: Name of the function to be distributed. The name must include the function’s parameter types in parentheses, because multiple functions can have the same name in PostgreSQL. For instance, 'foo(int)' is different from 'foo(int, text)'.

distribution_arg_name: (Optional) The argument name by which to distribute. For convenience (or if the function arguments do not have names), a positional placeholder is allowed, such as '$1'. If this parameter is not specified, then the function named by function_name is merely created on the workers. If worker nodes are added in the future the function will automatically be created there too.

colocate_with: (Optional) When the distributed function reads or writes to a distributed table (or more generally Co-Locating Tables), be sure to name that table using the colocate_with parameter. This ensures that each invocation of the function runs on the worker node containing relevant shards.

Return Value

N/A

Example

-- an example function which updates a hypothetical
-- event_responses table which itself is distributed by event_id
CREATE OR REPLACE FUNCTION
  register_for_event(p_event_id int, p_user_id int)
RETURNS void LANGUAGE plpgsql AS $fn$
BEGIN
  INSERT INTO event_responses VALUES ($1, $2, 'yes')
  ON CONFLICT (event_id, user_id)
  DO UPDATE SET response = EXCLUDED.response;
END;
$fn$;

-- distribute the function to workers, using the p_event_id argument
-- to determine which shard each invocation affects, and explicitly
-- colocating with event_responses which the function updates
SELECT create_distributed_function(
  'register_for_event(int, int)', 'p_event_id',
  colocate_with := 'event_responses'
);

master_create_distributed_table

Note

This function is deprecated, and replaced by create_distributed_table.

The master_create_distributed_table() function is used to define a distributed table. This function takes in a table name, the distribution column and distribution method and inserts appropriate metadata to mark the table as distributed.

Arguments

table_name: Name of the table which needs to be distributed.

distribution_column: The column on which the table is to be distributed.

distribution_method: The method according to which the table is to be distributed. Permissible values are append or hash.

Return Value

N/A

Example

This example informs the database that the github_events table should be distributed by hash on the repo_id column.

SELECT master_create_distributed_table('github_events', 'repo_id', 'hash');

master_create_worker_shards

Note

This function is deprecated, and replaced by create_distributed_table.

The master_create_worker_shards() function creates a specified number of worker shards with the desired replication factor for a hash distributed table. While doing so, the function also assigns a portion of the hash token space (which spans between -2 Billion and 2 Billion) to each shard. Once all shards are created, this function saves all distributed metadata on the coordinator.

Arguments

table_name: Name of hash distributed table for which shards are to be created.

shard_count: Number of shards to create.

replication_factor: Desired replication factor for each shard.

Return Value

N/A

Example

This example usage would create a total of 16 shards for the github_events table where each shard owns a portion of a hash token space and gets replicated on 2 workers.

SELECT master_create_worker_shards('github_events', 16, 2);

master_create_empty_shard

The master_create_empty_shard() function can be used to create an empty shard for an append distributed table. Behind the covers, the function first selects shard_replication_factor workers to create the shard on. Then, it connects to the workers and creates empty placements for the shard on the selected workers. Finally, the metadata is updated for these placements on the coordinator to make these shards visible to future queries. The function errors out if it is unable to create the desired number of shard placements.

Arguments

table_name: Name of the append distributed table for which the new shard is to be created.

Return Value

shard_id: The function returns the unique id assigned to the newly created shard.

Example

This example creates an empty shard for the github_events table. The shard id of the created shard is 102089.

SELECT * from master_create_empty_shard('github_events');
 master_create_empty_shard
---------------------------
                102089
(1 row)

Table and Shard DML

master_append_table_to_shard

The master_append_table_to_shard() function can be used to append a PostgreSQL table’s contents to a shard of an append distributed table. Behind the covers, the function connects to each of the workers which have a placement of that shard and appends the contents of the table to each of them. Then, the function updates metadata for the shard placements on the basis of whether the append succeeded or failed on each of them.

If the function is able to successfully append to at least one shard placement, the function will return successfully. It will also mark any placement to which the append failed as INACTIVE so that any future queries do not consider that placement. If the append fails for all placements, the function quits with an error (as no data was appended). In this case, the metadata is left unchanged.

Arguments

shard_id: Id of the shard to which the contents of the table have to be appended.

source_table_name: Name of the PostgreSQL table whose contents have to be appended.

source_node_name: DNS name of the node on which the source table is present (“source” node).

source_node_port: The port on the source worker node on which the database server is listening.

Return Value

shard_fill_ratio: The function returns the fill ratio of the shard which is defined as the ratio of the current shard size to the configuration parameter shard_max_size.

Example

This example appends the contents of the github_events_local table to the shard having shard id 102089. The table github_events_local is present on the database running on the node master-101 on port number 5432. The function returns the ratio of the the current shard size to the maximum shard size, which is 0.1 indicating that 10% of the shard has been filled.

SELECT * from master_append_table_to_shard(102089,'github_events_local','master-101', 5432);
 master_append_table_to_shard
------------------------------
                 0.100548
(1 row)

master_apply_delete_command

The master_apply_delete_command() function is used to delete shards which match the criteria specified by the delete command on an append distributed table. This function deletes a shard only if all rows in the shard match the delete criteria. As the function uses shard metadata to decide whether or not a shard needs to be deleted, it requires the WHERE clause in the DELETE statement to be on the distribution column. If no condition is specified, then all shards of that table are deleted.

Behind the covers, this function connects to all the worker nodes which have shards matching the delete criteria and sends them a command to drop the selected shards. Then, the function updates the corresponding metadata on the coordinator. If the function is able to successfully delete a shard placement, then the metadata for it is deleted. If a particular placement could not be deleted, then it is marked as TO DELETE. The placements which are marked as TO DELETE are not considered for future queries and can be cleaned up later.

Arguments

delete_command: valid SQL DELETE command

Return Value

deleted_shard_count: The function returns the number of shards which matched the criteria and were deleted (or marked for deletion). Note that this is the number of shards and not the number of shard placements.

Example

The first example deletes all the shards for the github_events table since no delete criteria is specified. In the second example, only the shards matching the criteria (3 in this case) are deleted.

SELECT * from master_apply_delete_command('DELETE FROM github_events');
 master_apply_delete_command
-----------------------------
                           5
(1 row)

SELECT * from master_apply_delete_command('DELETE FROM github_events WHERE review_date < ''2009-03-01''');
 master_apply_delete_command
-----------------------------
                           3
(1 row)

Metadata / Configuration Information

master_add_node

The master_add_node() function registers a new node addition in the cluster in the Citus metadata table pg_dist_node. It also copies reference tables to the new node.

Arguments

node_name: DNS name or IP address of the new node to be added.

node_port: The port on which PostgreSQL is listening on the worker node.

group_id: A group of one primary server and zero or more secondary servers, relevant only for streaming replication. Default -1

node_role: Whether it is ‘primary’ or ‘secondary’. Default ‘primary’

node_cluster: The cluster name. Default ‘default’

Return Value

The nodeid column from the newly inserted row in pg_dist_node.

Example

select * from master_add_node('new-node', 12345);
 master_add_node
-----------------
               7
(1 row)

master_update_node

The master_update_node() function changes the hostname and port for a node registered in the Citus metadata table pg_dist_node.

Arguments

node_id: id from the pg_dist_node table.

node_name: updated DNS name or IP address for the node.

node_port: the port on which PostgreSQL is listening on the worker node.

Return Value

N/A

Example

select * from master_update_node(123, 'new-address', 5432);

master_set_node_property

The master_set_node_property() function changes properties in the Citus metadata table pg_dist_node. Currently it can change only the shouldhaveshards property.

Arguments

node_name: DNS name or IP address for the node.

node_port: the port on which PostgreSQL is listening on the worker node.

property: the column to change in pg_dist_node, currently only shouldhaveshard is supported.

value: the new value for the column.

Return Value

N/A

Example

SELECT * FROM master_set_node_property('localhost', 5433, 'shouldhaveshards', false);

master_add_inactive_node

The master_add_inactive_node function, similar to master_add_node, registers a new node in pg_dist_node. However it marks the new node as inactive, meaning no shards will be placed there. Also it does not copy reference tables to the new node.

Arguments

node_name: DNS name or IP address of the new node to be added.

node_port: The port on which PostgreSQL is listening on the worker node.

group_id: A group of one primary server and zero or more secondary servers, relevant only for streaming replication. Default -1

node_role: Whether it is ‘primary’ or ‘secondary’. Default ‘primary’

node_cluster: The cluster name. Default ‘default’

Return Value

The nodeid column from the newly inserted row in pg_dist_node.

Example

select * from master_add_inactive_node('new-node', 12345);
 master_add_inactive_node
--------------------------
                        7
(1 row)

master_activate_node

The master_activate_node function marks a node as active in the Citus metadata table pg_dist_node and copies reference tables to the node. Useful for nodes added via master_add_inactive_node.

Arguments

node_name: DNS name or IP address of the new node to be added.

node_port: The port on which PostgreSQL is listening on the worker node.

Return Value

The nodeid column from the newly inserted row in pg_dist_node.

Example

select * from master_activate_node('new-node', 12345);
 master_activate_node
----------------------
                    7
(1 row)

master_disable_node

The master_disable_node function is the opposite of master_activate_node. It marks a node as inactive in the Citus metadata table pg_dist_node, removing it from the cluster temporarily. The function also deletes all reference table placements from the disabled node. To reactivate the node, just run master_activate_node again.

Arguments

node_name: DNS name or IP address of the node to be disabled.

node_port: The port on which PostgreSQL is listening on the worker node.

Return Value

N/A

Example

select * from master_disable_node('new-node', 12345);

master_add_secondary_node

The master_add_secondary_node() function registers a new secondary node in the cluster for an existing primary node. It updates the Citus metadata table pg_dist_node.

Arguments

node_name: DNS name or IP address of the new node to be added.

node_port: The port on which PostgreSQL is listening on the worker node.

primary_name: DNS name or IP address of the primary node for this secondary.

primary_port: The port on which PostgreSQL is listening on the primary node.

node_cluster: The cluster name. Default ‘default’

Return Value

The nodeid column for the secondary node, inserted row in pg_dist_node.

Example

select * from master_add_secondary_node('new-node', 12345, 'primary-node', 12345);
 master_add_secondary_node
---------------------------
                         7
(1 row)

master_remove_node

The master_remove_node() function removes the specified node from the pg_dist_node metadata table. This function will error out if there are existing shard placements on this node. Thus, before using this function, the shards will need to be moved off that node.

Arguments

node_name: DNS name of the node to be removed.

node_port: The port on which PostgreSQL is listening on the worker node.

Return Value

N/A

Example

select master_remove_node('new-node', 12345);
 master_remove_node
--------------------

(1 row)

master_get_active_worker_nodes

The master_get_active_worker_nodes() function returns a list of active worker host names and port numbers.

Arguments

N/A

Return Value

List of tuples where each tuple contains the following information:

node_name: DNS name of the worker node

node_port: Port on the worker node on which the database server is listening

Example

SELECT * from master_get_active_worker_nodes();
 node_name | node_port
-----------+-----------
 localhost |      9700
 localhost |      9702
 localhost |      9701

(3 rows)

master_get_table_metadata

The master_get_table_metadata() function can be used to return distribution related metadata for a distributed table. This metadata includes the relation id, storage type, distribution method, distribution column, replication count, maximum shard size and the shard placement policy for that table. Behind the covers, this function queries Citus metadata tables to get the required information and concatenates it into a tuple before returning it to the user.

Arguments

table_name: Name of the distributed table for which you want to fetch metadata.

Return Value

A tuple containing the following information:

logical_relid: Oid of the distributed table. This values references the relfilenode column in the pg_class system catalog table.

part_storage_type: Type of storage used for the table. May be ‘t’ (standard table), ‘f’ (foreign table) or ‘c’ (columnar table).

part_method: Distribution method used for the table. May be ‘a’ (append), or ‘h’ (hash).

part_key: Distribution column for the table.

part_replica_count: Current shard replication count.

part_max_size: Current maximum shard size in bytes.

part_placement_policy: Shard placement policy used for placing the table’s shards. May be 1 (local-node-first) or 2 (round-robin).

Example

The example below fetches and displays the table metadata for the github_events table.

SELECT * from master_get_table_metadata('github_events');
 logical_relid | part_storage_type | part_method | part_key | part_replica_count | part_max_size | part_placement_policy
---------------+-------------------+-------------+----------+--------------------+---------------+-----------------------
         24180 | t                 | h           | repo_id  |                  2 |    1073741824 |                     2
(1 row)

get_shard_id_for_distribution_column

Citus assigns every row of a distributed table to a shard based on the value of the row’s distribution column and the table’s method of distribution. In most cases the precise mapping is a low-level detail that the database administrator can ignore. However it can be useful to determine a row’s shard, either for manual database maintenance tasks or just to satisfy curiosity. The get_shard_id_for_distribution_column function provides this info for hash- and range-distributed tables as well as reference tables. It does not work for the append distribution.

Arguments

table_name: The distributed table.

distribution_value: The value of the distribution column.

Return Value

The shard id Citus associates with the distribution column value for the given table.

Example

SELECT get_shard_id_for_distribution_column('my_table', 4);

 get_shard_id_for_distribution_column
--------------------------------------
                               540007
(1 row)

column_to_column_name

Translates the partkey column of pg_dist_partition into a textual column name. This is useful to determine the distribution column of a distributed table.

For a more detailed discussion, see Finding the distribution column for a table.

Arguments

table_name: The distributed table.

column_var_text: The value of partkey in the pg_dist_partition table.

Return Value

The name of table_name‘s distribution column.

Example

-- get distribution column name for products table

SELECT column_to_column_name(logicalrelid, partkey) AS dist_col_name
  FROM pg_dist_partition
 WHERE logicalrelid='products'::regclass;

Output:

┌───────────────┐
│ dist_col_name │
├───────────────┤
│ company_id    │
└───────────────┘

citus_relation_size

Get the disk space used by all the shards of the specified distributed table. This includes the size of the “main fork,” but excludes the visibility map and free space map for the shards.

Arguments

logicalrelid: the name of a distributed table.

Return Value

Size in bytes as a bigint.

Example

SELECT pg_size_pretty(citus_relation_size('github_events'));
pg_size_pretty
--------------
23 MB

citus_table_size

Get the disk space used by all the shards of the specified distributed table, excluding indexes (but including TOAST, free space map, and visibility map).

Arguments

logicalrelid: the name of a distributed table.

Return Value

Size in bytes as a bigint.

Example

SELECT pg_size_pretty(citus_table_size('github_events'));
pg_size_pretty
--------------
37 MB

citus_total_relation_size

Get the total disk space used by the all the shards of the specified distributed table, including all indexes and TOAST data.

Arguments

logicalrelid: the name of a distributed table.

Return Value

Size in bytes as a bigint.

Example

SELECT pg_size_pretty(citus_total_relation_size('github_events'));
pg_size_pretty
--------------
73 MB

citus_stat_statements_reset

Removes all rows from citus_stat_statements. Note that this works independently from pg_stat_statements_reset(). To reset all stats, call both functions.

Arguments

N/A

Return Value

None

Cluster Management And Repair Functions

master_copy_shard_placement

If a shard placement fails to be updated during a modification command or a DDL operation, then it gets marked as inactive. The master_copy_shard_placement function can then be called to repair an inactive shard placement using data from a healthy placement.

To repair a shard, the function first drops the unhealthy shard placement and recreates it using the schema on the coordinator. Once the shard placement is created, the function copies data from the healthy placement and updates the metadata to mark the new shard placement as healthy. This function ensures that the shard will be protected from any concurrent modifications during the repair.

Arguments

shard_id: Id of the shard to be repaired.

source_node_name: DNS name of the node on which the healthy shard placement is present (“source” node).

source_node_port: The port on the source worker node on which the database server is listening.

target_node_name: DNS name of the node on which the invalid shard placement is present (“target” node).

target_node_port: The port on the target worker node on which the database server is listening.

Return Value

N/A

Example

The example below will repair an inactive shard placement of shard 12345 which is present on the database server running on ‘bad_host’ on port 5432. To repair it, it will use data from a healthy shard placement present on the server running on ‘good_host’ on port 5432.

SELECT master_copy_shard_placement(12345, 'good_host', 5432, 'bad_host', 5432);

master_move_shard_placement

Note

The master_move_shard_placement function is a part of Citus Enterprise. Please contact us to obtain this functionality.

This function moves a given shard (and shards co-located with it) from one node to another. It is typically used indirectly during shard rebalancing rather than being called directly by a database administrator.

There are two ways to move the data: blocking or nonblocking. The blocking approach means that during the move all modifications to the shard are paused. The second way, which avoids blocking shard writes, relies on Postgres 10 logical replication.

After a successful move operation, shards in the source node get deleted. If the move fails at any point, this function throws an error and leaves the source and target nodes unchanged.

Arguments

shard_id: Id of the shard to be moved.

source_node_name: DNS name of the node on which the healthy shard placement is present (“source” node).

source_node_port: The port on the source worker node on which the database server is listening.

target_node_name: DNS name of the node on which the invalid shard placement is present (“target” node).

target_node_port: The port on the target worker node on which the database server is listening.

shard_transfer_mode: (Optional) Specify the method of replication, whether to use PostgreSQL logical replication or a cross-worker COPY command. The possible values are:

  • auto: Require replica identity if logical replication is possible, otherwise use legacy behaviour (e.g. for shard repair, PostgreSQL 9.6). This is the default value.
  • force_logical: Use logical replication even if the table doesn’t have a replica identity. Any concurrent update/delete statements to the table will fail during replication.
  • block_writes: Use COPY (blocking writes) for tables lacking primary key or replica identity.

Return Value

N/A

Example

SELECT master_move_shard_placement(12345, 'from_host', 5432, 'to_host', 5432);

rebalance_table_shards

Note

The rebalance_table_shards function is a part of Citus Enterprise. Please contact us to obtain this functionality.

The rebalance_table_shards() function moves shards of the given table to make them evenly distributed among the workers. The function first calculates the list of moves it needs to make in order to ensure that the cluster is balanced within the given threshold. Then, it moves shard placements one by one from the source node to the destination node and updates the corresponding shard metadata to reflect the move.

Every shard is assigned a cost when determining whether shards are “evenly distributed.” By default each shard has the same cost (a value of 1), so distributing to equalize the cost across workers is the same as equalizing the number of shards on each. The constant cost strategy is called “by_shard_count” and is the default rebalancing strategy.

The default strategy is appropriate under these circumstances:

  1. The shards are roughly the same size
  2. The shards get roughly the same amount of traffic
  3. Worker nodes are all the same size/type
  4. Shards haven’t been pinned to particular workers

If any of these assumptions don’t hold, then the default rebalancing can result in a bad plan. In this case you may customize the strategy, using the rebalance_strategy parameter.

It’s advisable to call get_rebalance_table_shards_plan before running rebalance_table_shards, to see and verify the actions to be performed.

Arguments

table_name: (Optional) The name of the table whose shards need to be rebalanced. If NULL, then rebalance all existing colocation groups.

threshold: (Optional) A float number between 0.0 and 1.0 which indicates the maximum difference ratio of node utilization from average utilization. For example, specifying 0.1 will cause the shard rebalancer to attempt to balance all nodes to hold the same number of shards ±10%. Specifically, the shard rebalancer will try to converge utilization of all worker nodes to the (1 - threshold) * average_utilization ... (1 + threshold) * average_utilization range.

max_shard_moves: (Optional) The maximum number of shards to move.

excluded_shard_list: (Optional) Identifiers of shards which shouldn’t be moved during the rebalance operation.

shard_transfer_mode: (Optional) Specify the method of replication, whether to use PostgreSQL logical replication or a cross-worker COPY command. The possible values are:

  • auto: Require replica identity if logical replication is possible, otherwise use legacy behaviour (e.g. for shard repair, PostgreSQL 9.6). This is the default value.
  • force_logical: Use logical replication even if the table doesn’t have a replica identity. Any concurrent update/delete statements to the table will fail during replication.
  • block_writes: Use COPY (blocking writes) for tables lacking primary key or replica identity.

drain_only: (Optional) When true, move shards off worker nodes who have shouldhaveshards set to false in Worker node table; move no other shards.

rebalance_strategy: (Optional) the name of a strategy in Rebalancer strategy table. If this argument is omitted, the function chooses the default strategy, as indicated in the table.

Return Value

N/A

Example

The example below will attempt to rebalance the shards of the github_events table within the default threshold.

SELECT rebalance_table_shards('github_events');

This example usage will attempt to rebalance the github_events table without moving shards with id 1 and 2.

SELECT rebalance_table_shards('github_events', excluded_shard_list:='{1,2}');

get_rebalance_table_shards_plan

Note

The get_rebalance_table_shards_plan function is a part of Citus Enterprise. Please contact us to obtain this functionality.

Output the planned shard movements of rebalance_table_shards without performing them. While it’s unlikely, get_rebalance_table_shards_plan can output a slightly different plan than what a rebalance_table_shards call with the same arguments will do. This could happen because they are not executed at the same time, so facts about the cluster – e.g. disk space – might differ between the calls.

Arguments

The same arguments as rebalance_table_shards: relation, threshold, max_shard_moves, excluded_shard_list, and drain_only. See documentation of that function for the arguments’ meaning.

Return Value

Tuples containing these columns:

  • table_name: The table whose shards would move
  • shardid: The shard in question
  • shard_size: Size in bytes
  • sourcename: Hostname of the source node
  • sourceport: Port of the source node
  • targetname: Hostname of the destination node
  • targetport: Port of the destination node

get_rebalance_progress

Note

The get_rebalance_progress() function is a part of Citus Enterprise. Please contact us to obtain this functionality.

Once a shard rebalance begins, the get_rebalance_progress() function lists the progress of every shard involved. It monitors the moves planned and executed by rebalance_table_shards().

Arguments

N/A

Return Value

Tuples containing these columns:

  • sessionid: Postgres PID of the rebalance monitor
  • table_name: The table whose shards are moving
  • shardid: The shard in question
  • shard_size: Size in bytes
  • sourcename: Hostname of the source node
  • sourceport: Port of the source node
  • targetname: Hostname of the destination node
  • targetport: Port of the destination node
  • progress: 0 = waiting to be moved; 1 = moving; 2 = complete

Example

SELECT * FROM get_rebalance_progress();
┌───────────┬────────────┬─────────┬────────────┬───────────────┬────────────┬───────────────┬────────────┬──────────┐
│ sessionid │ table_name │ shardid │ shard_size │  sourcename   │ sourceport │  targetname   │ targetport │ progress │
├───────────┼────────────┼─────────┼────────────┼───────────────┼────────────┼───────────────┼────────────┼──────────┤
│      7083 │ foo        │  102008 │    1204224 │ n1.foobar.com │       5432 │ n4.foobar.com │       5432 │        0 │
│      7083 │ foo        │  102009 │    1802240 │ n1.foobar.com │       5432 │ n4.foobar.com │       5432 │        0 │
│      7083 │ foo        │  102018 │     614400 │ n2.foobar.com │       5432 │ n4.foobar.com │       5432 │        1 │
│      7083 │ foo        │  102019 │       8192 │ n3.foobar.com │       5432 │ n4.foobar.com │       5432 │        2 │
└───────────┴────────────┴─────────┴────────────┴───────────────┴────────────┴───────────────┴────────────┴──────────┘

citus_add_rebalance_strategy

Append a row to the pg_dist_rebalance_strategy.

Arguments

For more about these arguments, see the corresponding column values in Rebalancer strategy table.

name: identifier for the new strategy

shard_cost_function: identifies the function used to determine the “cost” of each shard

node_capacity_function: identifies the function to measure node capacity

shard_allowed_on_node_function: identifies the function which determines which shards can be placed on which nodes

default_threshold: a floating point threshold that tunes how precisely the cumulative shard cost should be balanced between nodes

minimum_threshold: (Optional) a safeguard column that holds the minimum value allowed for the threshold argument of rebalance_table_shards(). Its default value is 0

Return Value

N/A

citus_set_default_rebalance_strategy

Note

The citus_set_default_rebalance_strategy function is a part of Citus Enterprise. Please contact us to obtain this functionality.

Update the Rebalancer strategy table table, changing the strategy named by its argument to be the default chosen when rebalancing shards.

Arguments

name: the name of the strategy in pg_dist_rebalance_strategy

Return Value

N/A

Example

SELECT citus_set_default_rebalance_strategy('by_disk_size');

citus_remote_connection_stats

The citus_remote_connection_stats() function shows the number of active connections to each remote node.

Arguments

N/A

Example

SELECT * from citus_remote_connection_stats();
.
    hostname    | port | database_name | connection_count_to_node
----------------+------+---------------+--------------------------
 citus_worker_1 | 5432 | postgres      |                        3
(1 row)

master_drain_node

Note

The master_drain_node function is a part of Citus Enterprise. Please contact us to obtain this functionality.

The master_drain_node() function moves shards off the designated node and onto other nodes who have shouldhaveshards set to true in Worker node table. This function is designed to be called prior to removing a node from the cluster, i.e. turning the node’s physical server off.

Arguments

nodename: The hostname name of the node to be drained.

nodeport: The port number of the node to be drained.

shard_transfer_mode: (Optional) Specify the method of replication, whether to use PostgreSQL logical replication or a cross-worker COPY command. The possible values are:

  • auto: Require replica identity if logical replication is possible, otherwise use legacy behaviour (e.g. for shard repair, PostgreSQL 9.6). This is the default value.
  • force_logical: Use logical replication even if the table doesn’t have a replica identity. Any concurrent update/delete statements to the table will fail during replication.
  • block_writes: Use COPY (blocking writes) for tables lacking primary key or replica identity.

rebalance_strategy: (Optional) the name of a strategy in Rebalancer strategy table. If this argument is omitted, the function chooses the default strategy, as indicated in the table.

Return Value

N/A

Example

Here are the typical steps to remove a single node (for example ‘10.0.0.1’ on a standard PostgreSQL port):

  1. Drain the node.

    SELECT * from master_drain_node('10.0.0.1', 5432);
    
  2. Wait until the command finishes

  3. Remove the node

When draining multiple nodes it’s recommended to use rebalance_table_shards instead. Doing so allows Citus to plan ahead and move shards the minimum number of times.

  1. Run this for each node that you want to remove:

    SELECT * FROM master_set_node_property(node_hostname, node_port, 'shouldhaveshards', false);
    
  2. Drain them all at once with rebalance_table_shards:

    SELECT * FROM rebalance_table_shards(drain_only := true);
    
  3. Wait until the draining rebalance finishes

  4. Remove the nodes

replicate_table_shards

Note

The replicate_table_shards function is a part of Citus Enterprise. Please contact us to obtain this functionality.

The replicate_table_shards() function replicates the under-replicated shards of the given table. The function first calculates the list of under-replicated shards and locations from which they can be fetched for replication. The function then copies over those shards and updates the corresponding shard metadata to reflect the copy.

Arguments

table_name: The name of the table whose shards need to be replicated.

shard_replication_factor: (Optional) The desired replication factor to achieve for each shard.

max_shard_copies: (Optional) Maximum number of shards to copy to reach the desired replication factor.

excluded_shard_list: (Optional) Identifiers of shards which shouldn’t be copied during the replication operation.

Return Value

N/A

Examples

The example below will attempt to replicate the shards of the github_events table to shard_replication_factor.

SELECT replicate_table_shards('github_events');

This example will attempt to bring the shards of the github_events table to the desired replication factor with a maximum of 10 shard copies. This means that the rebalancer will copy only a maximum of 10 shards in its attempt to reach the desired replication factor.

SELECT replicate_table_shards('github_events', max_shard_copies:=10);

isolate_tenant_to_new_shard

Note

The isolate_tenant_to_new_shard function is a part of Citus Enterprise. Please contact us to obtain this functionality.

This function creates a new shard to hold rows with a specific single value in the distribution column. It is especially handy for the multi-tenant Citus use case, where a large tenant can be placed alone on its own shard and ultimately its own physical node.

For a more in-depth discussion, see Tenant Isolation.

Arguments

table_name: The name of the table to get a new shard.

tenant_id: The value of the distribution column which will be assigned to the new shard.

cascade_option: (Optional) When set to “CASCADE,” also isolates a shard from all tables in the current table’s Co-Locating Tables.

Return Value

shard_id: The function returns the unique id assigned to the newly created shard.

Examples

Create a new shard to hold the lineitems for tenant 135:

SELECT isolate_tenant_to_new_shard('lineitem', 135);
┌─────────────────────────────┐
│ isolate_tenant_to_new_shard │
├─────────────────────────────┤
│                      102240 │
└─────────────────────────────┘

citus_create_restore_point

Temporarily blocks writes to the cluster, and creates a named restore point on all nodes. This function is similar to pg_create_restore_point, but applies to all nodes and makes sure the restore point is consistent across them. This function is well suited to doing point-in-time recovery, and cluster forking.

Arguments

name: The name of the restore point to create.

Return Value

coordinator_lsn: Log sequence number of the restore point in the coordinator node WAL.

Examples

select citus_create_restore_point('foo');
┌────────────────────────────┐
│ citus_create_restore_point │
├────────────────────────────┤
│ 0/1EA2808                  │
└────────────────────────────┘