Frequently Asked Questions
Can I create primary keys on distributed tables?
Currently Citus imposes primary key constraint only if the distribution column is a part of the primary key. This assures that the constraint needs to be checked only on one shard to ensure uniqueness.
How do I add nodes to an existing Citus cluster?
On the Citus managed service with Azure Cosmos DB for PostgreSQL (formerly known as Hyperscale (Citus)), adding nodes is quite easy: all it takes is a few clicks in the Azure portal. With Citus open source, you can add nodes manually by calling the citus_add_node UDF with the hostname (or IP address) and port number of the new node.
Either way, after adding a node to an existing cluster, the new node will not contain any data (shards). Citus will start assigning any newly created shards to this node. To rebalance existing shards from the older nodes to the new node, Citus provides an open source shard rebalancer utility. You can find more information in the Rebalance Shards without Downtime section.
How does Citus handle failure of a worker node?
Citus uses PostgreSQL’s streaming replication to replicate the entire worker-node as-is. It replicates worker nodes by continuously streaming their WAL records to a standby. You can configure streaming replication on-premise yourself by consulting the PostgreSQL replication documentation.
How does Citus handle failover of the coordinator node?
As the Citus coordinator node is similar to a standard PostgreSQL server, regular PostgreSQL synchronous replication and failover can be used to provide higher availability of the coordinator node. Many of our customers use synchronous replication in this way to add resilience against coordinator node failure. You can find more information about handling Coordinator Node Failures.
Are there any PostgreSQL features not supported by Citus?
Since Citus provides distributed functionality by extending PostgreSQL, it uses the standard PostgreSQL SQL constructs. The vast majority of queries are supported, even when they combine data across the network from multiple database nodes. This includes transactional semantics across nodes. Currently all SQL is supported except:
SELECT … FOR UPDATE
What’s more, Citus has 100% SQL support for queries which access a single node in the database cluster. These queries are common, for instance, in multi-tenant applications where different nodes store different tenants (see When to Use Citus).
Remember that – even with this extensive SQL coverage – data modeling can have a significant impact on query performance. See the section on Query Processing for details on how Citus executes queries.
How do I choose the shard count when I hash-partition my data?
One of the choices when first distributing a table is its shard count. This setting can be set differently for each co-location group, and the optimal value depends on use-case. It is possible, but difficult, to change the count after cluster creation, so use these guidelines to choose the right size.
In the Multi-Tenant SaaS Database use-case we recommend choosing between 32 - 128 shards. For smaller workloads say <100GB, you could start with 32 shards and for larger workloads you could choose 64 or 128. This means that you have the leeway to scale from 32 to 128 worker machines.
In the Real-Time Analytics use-case, shard count should be related to the total number of cores on the workers. To ensure maximum parallelism, you should create enough shards on each node such that there is at least one shard per CPU core. We typically recommend creating a high number of initial shards, e.g. 2x or 4x the number of current CPU cores. This allows for future scaling if you add more workers and CPU cores.
To choose a shard count for a table you wish to distribute, update the
citus.shard_count variable. This affects subsequent calls to create_distributed_table. For example
SET citus.shard_count = 64; -- any tables distributed at this point will have -- sixty-four shards
For more guidance on this topic, see Choosing Cluster Size.
How do I change the shard count for a hash partitioned table?
Citus has a function called alter_distributed_table that can change the shard count of a distributed table.
How does citus support count(distinct) queries?
Citus can evaluate count(distinct) aggregates both in and across worker nodes. When aggregating on a table’s distribution column, Citus can push the counting down inside worker nodes and total the results. Otherwise it can pull distinct rows to the coordinator and calculate there. If transferring data to the coordinator is too expensive, fast approximate counts are also available. More details in Count (Distinct) Aggregates.
In which situations are uniqueness constraints supported on distributed tables?
Citus is able to enforce a primary key or uniqueness constraint only when the constrained columns contain the distribution column. In particular this means that if a single column constitutes the primary key then it has to be the distribution column as well.
This restriction allows Citus to localize a uniqueness check to a single shard and let PostgreSQL on the worker node do the check efficiently.
How do I create database roles, functions, extensions etc in a Citus cluster?
Certain commands, when run on the coordinator node, do not get propagated to the workers:
ALTER … SET SCHEMA
ALTER TABLE ALL IN TABLESPACE
CREATE TABLE(see Table Types)
For the other types of objects above, create them explicitly on all nodes. Citus provides a function to execute queries across all workers:
SELECT run_command_on_workers($cmd$ /* the command to run */ CREATE ROLE ... $cmd$);
In the future Citus will automatically propagate more kinds of objects. The advantage of automatic propagation is that Citus will automatically create a copy on any newly added worker nodes (see Distributed object table for more about that.)
What if a worker node’s address changes?
If the hostname or IP address of a worker changes, you need to let the coordinator know using citus_update_node:
-- update worker node metadata on the coordinator -- (remember to replace 'old-address' and 'new-address' -- with the actual values for your situation) select citus_update_node(nodeid, 'new-address', nodeport) from pg_dist_node where nodename = 'old-address';
Until you execute this update, the coordinator will not be able to communicate with that worker for queries.
Which shard contains data for a particular tenant?
Citus provides UDFs and metadata tables to determine the mapping of a distribution column value to a particular shard, and the shard placement on a worker node. See Finding which shard contains data for a specific tenant for more details.
I forgot the distribution column of a table, how do I find it?
The Citus coordinator node metadata tables contain this information. See Finding the distribution column for a table.
Can I distribute a table by multiple keys?
No, you must choose a single column per table as the distribution column. A common scenario where people want to distribute by two columns is for timeseries data. However, for this case we recommend using a hash distribution on a non-time column, and combining this with PostgreSQL partitioning on the time column, as described in Timeseries Data.
Why does pg_relation_size report zero bytes for a distributed table?
The data in distributed tables lives on the worker nodes (in shards), not on the coordinator. A true measure of distributed table size is obtained as a sum of shard sizes. Citus provides helper functions to query this information. See Determining Table and Relation Size to learn more.
Why am I seeing an error about max_intermediate_result_size?
Citus has to use more than one step to run some queries having subqueries or CTEs. Using Subquery/CTE Push-Pull Execution, it pushes subquery results to all worker nodes for use by the main query. If these results are too large, this might cause unacceptable network overhead, or even insufficient storage space on the coordinator node which accumulates and distributes the results.
Citus has a configurable setting,
citus.max_intermediate_result_size to specify a subquery result size threshold at which the query will be canceled. If you run into the error, it looks like:
ERROR: the intermediate result size exceeds citus.max_intermediate_result_size (currently 1 GB) DETAIL: Citus restricts the size of intermediate results of complex subqueries and CTEs to avoid accidentally pulling large result sets into once place. HINT: To run the current query, set citus.max_intermediate_result_size to a higher value or -1 to disable.
As the error message suggests, you can (cautiously) increase this limit by altering the variable:
SET citus.max_intermediate_result_size = '3GB';
Can I run Citus on Microsoft Azure?
Yes, Citus is available as a managed service with Azure Cosmos DB for PostgreSQL, formerly known as Hyperscale (Citus) in Azure Database for PostgreSQL.
Can I shard by schema on Citus for multi-tenant applications?
It turns out that while storing each tenant’s information in a separate schema can be an attractive way to start when dealing with tenants, it leads to problems down the road. In Citus we partition by the tenant_id, and a shard can contain data from several tenants. To learn more about the reason for this design, see our article Lessons learned from PostgreSQL schema sharding.
How does cstore_fdw work with Citus?
The cstore_fdw extension is no longer needed on PostgreSQL 12 and above, because Columnar Storage is now implemented directly in Citus. Unlike cstore_fdw, Citus’ columnar tables support transactional semantics, replication, and pg_upgrade. Citus’ query parallelization, seamless sharding, and HA benefits combine powerfully with the superior compression and I/O utilization of columnar storage for large dataset archival and reporting.
What happened to pg_shard?
The pg_shard extension is deprecated and no longer supported.
Starting with the open-source release of Citus v5.x, pg_shard’s codebase has been merged into Citus to offer you a unified solution which provides the advanced distributed query planning previously only enjoyed by CitusDB customers while preserving the simple and transparent sharding and real-time writes and reads pg_shard brought to the PostgreSQL ecosystem. Our flagship product, Citus, provides a superset of the functionality of pg_shard and we have migration steps to help existing users to perform a drop-in replacement. Please contact us for more information.