Citus Tables and Views

Coordinator Metadata

Citus divides each distributed table into multiple logical shards based on the distribution column. The coordinator then maintains metadata tables to track statistics and information about the health and location of these shards. In this section, we describe each of these metadata tables and their schema. You can view and query these tables using SQL after logging into the coordinator node.

Partition table

The pg_dist_partition table stores metadata about which tables in the database are distributed. For each distributed table, it also stores information about the distribution method and detailed information about the distribution column.

SELECT * from pg_dist_partition;
 logicalrelid  | partmethod |                                                        partkey                                                         | colocationid | repmodel
---------------+------------+------------------------------------------------------------------------------------------------------------------------+--------------+----------
 github_events | h          | {VAR :varno 1 :varattno 4 :vartype 20 :vartypmod -1 :varcollid 0 :varlevelsup 0 :varnoold 1 :varoattno 4 :location -1} |            2 | c
 (1 row)

Shard table

The pg_dist_shard table stores metadata about individual shards of a table. This includes information about which distributed table the shard belongs to and statistics about the distribution column for that shard. For append distributed tables, these statistics correspond to min / max values of the distribution column. In case of hash distributed tables, they are hash token ranges assigned to that shard. These statistics are used for pruning away unrelated shards during SELECT queries.

SELECT * from pg_dist_shard;
 logicalrelid  | shardid | shardstorage | shardminvalue | shardmaxvalue
---------------+---------+--------------+---------------+---------------
 github_events |  102026 | t            | 268435456     | 402653183
 github_events |  102027 | t            | 402653184     | 536870911
 github_events |  102028 | t            | 536870912     | 671088639
 github_events |  102029 | t            | 671088640     | 805306367
 (4 rows)

Shard Storage Types

The shardstorage column in pg_dist_shard indicates the type of storage used for the shard. A brief overview of different shard storage types and their representation is below.

Shard information view

In addition to the low-level shard metadata table described above, Citus provides a citus_shards view to easily check:

• Where each shard is (node, and port),

• What kind of table it belongs to, and

• Its size

This view helps you inspect shards to find, among other things, any size imbalances across nodes.

SELECT * FROM citus_shards;

.
 table_name | shardid | shard_name   | citus_table_type | colocation_id | nodename  | nodeport | shard_size
------------+---------+--------------+------------------+---------------+-----------+----------+------------
 dist       |  102170 | dist_102170  | distributed      |            34 | localhost |     9701 |   90677248
 dist       |  102171 | dist_102171  | distributed      |            34 | localhost |     9702 |   90619904
 dist       |  102172 | dist_102172  | distributed      |            34 | localhost |     9701 |   90701824
 dist       |  102173 | dist_102173  | distributed      |            34 | localhost |     9702 |   90693632
 ref        |  102174 | ref_102174   | reference        |             2 | localhost |     9701 |       8192
 ref        |  102174 | ref_102174   | reference        |             2 | localhost |     9702 |       8192
 dist2      |  102175 | dist2_102175 | distributed      |            34 | localhost |     9701 |     933888
 dist2      |  102176 | dist2_102176 | distributed      |            34 | localhost |     9702 |     950272
 dist2      |  102177 | dist2_102177 | distributed      |            34 | localhost |     9701 |     942080
 dist2      |  102178 | dist2_102178 | distributed      |            34 | localhost |     9702 |     933888

The colocation_id refers to the colocation group. For more info about citus_table_type, see Table Types.

Shard placement table

The pg_dist_placement table tracks the location of shard replicas on worker nodes. Each replica of a shard assigned to a specific node is called a shard placement. This table stores information about the health and location of each shard placement.

SELECT * from pg_dist_placement;
  placementid | shardid | shardstate | shardlength | groupid
 -------------+---------+------------+-------------+---------
            1 |  102008 |          1 |           0 |       1
            2 |  102008 |          1 |           0 |       2
            3 |  102009 |          1 |           0 |       2
            4 |  102009 |          1 |           0 |       3
            5 |  102010 |          1 |           0 |       3
            6 |  102010 |          1 |           0 |       4
            7 |  102011 |          1 |           0 |       4

Note

As of Citus 7.0 the analogous table pg_dist_shard_placement has been deprecated. It included the node name and port for each placement:

SELECT * from pg_dist_shard_placement;
  shardid | shardstate | shardlength | nodename  | nodeport | placementid
 ---------+------------+-------------+-----------+----------+-------------
   102008 |          1 |           0 | localhost |    12345 |           1
   102008 |          1 |           0 | localhost |    12346 |           2
   102009 |          1 |           0 | localhost |    12346 |           3
   102009 |          1 |           0 | localhost |    12347 |           4
   102010 |          1 |           0 | localhost |    12347 |           5
   102010 |          1 |           0 | localhost |    12345 |           6
   102011 |          1 |           0 | localhost |    12345 |           7

That information is now available by joining pg_dist_placement with pg_dist_node on the groupid. For compatibility Citus still provides pg_dist_shard_placement as a view. However, we recommend using the new, more normalized, tables when possible.

Shard Placement States

Citus manages shard health on a per-placement basis and automatically marks a placement as unavailable if leaving the placement in service would put the cluster in an inconsistent state. The shardstate column in the pg_dist_placement table is used to store the state of shard placements. A brief overview of different shard placement states and their representation is below.

Worker node table

The pg_dist_node table contains information about the worker nodes in the cluster.

SELECT * from pg_dist_node;
 nodeid | groupid | nodename  | nodeport | noderack | hasmetadata | isactive | noderole | nodecluster | metadatasynced | shouldhaveshards
--------+---------+-----------+----------+----------+-------------+----------+----------+-------------+----------------+------------------
      1 |       1 | localhost |    12345 | default  | f           | t        | primary  | default     | f              | t
      2 |       2 | localhost |    12346 | default  | f           | t        | primary  | default     | f              | t
      3 |       3 | localhost |    12347 | default  | f           | t        | primary  | default     | f              | t
(3 rows)

Distributed object table

The citus.pg_dist_object table contains a list of objects such as types and functions that have been created on the coordinator node and propagated to worker nodes. When an administrator adds new worker nodes to the cluster, Citus automatically creates copies of the distributed objects on the new nodes (in the correct order to satisfy object dependencies).

“Stable addresses” uniquely identify objects independently of a specific server. Citus tracks objects during a PostgreSQL upgrade using stable addresses created with the pg_identify_object_as_address() function.

Here’s an example of how create_distributed_function() adds entries to the citus.pg_dist_object table:

CREATE TYPE stoplight AS enum ('green', 'yellow', 'red');
CREATE OR REPLACE FUNCTION intersection()
RETURNS stoplight AS $$
DECLARE
        color stoplight;
BEGIN
        SELECT *
          FROM unnest(enum_range(NULL::stoplight)) INTO color
         ORDER BY random() LIMIT 1;
        RETURN color;
END;
$$ LANGUAGE plpgsql VOLATILE;
SELECT create_distributed_function('intersection()');
-- will have two rows, one for the TYPE and one for the FUNCTION
TABLE citus.pg_dist_object;

-[ RECORD 1 ]---------------+------
classid                     | 1247
objid                       | 16780
objsubid                    | 0
type                        |
object_names                |
object_args                 |
distribution_argument_index |
colocationid                |
-[ RECORD 2 ]---------------+------
classid                     | 1255
objid                       | 16788
objsubid                    | 0
type                        |
object_names                |
object_args                 |
distribution_argument_index |
colocationid                |

Citus tables view

The citus_tables view shows a summary of all tables managed by Citus (distributed and reference tables). The view combines information from Citus metadata tables for an easy, human-readable overview of these table properties:

• Table type

• Distribution column

• Colocation group id

• Human-readable size

• Shard count

• Owner (database user)

• Access method (heap or columnar)

Here’s an example:

SELECT * FROM citus_tables;

┌────────────┬──────────────────┬─────────────────────┬───────────────┬────────────┬─────────────┬─────────────┬───────────────┐
│ table_name │ citus_table_type │ distribution_column │ colocation_id │ table_size │ shard_count │ table_owner │ access_method │
├────────────┼──────────────────┼─────────────────────┼───────────────┼────────────┼─────────────┼─────────────┼───────────────┤
│ foo.test   │ distributed      │ test_column         │             1 │ 0 bytes    │          32 │ citus       │ heap          │
│ ref        │ reference        │ <none>              │             2 │ 24 GB      │           1 │ citus       │ heap          │
│ test       │ distributed      │ id                  │             1 │ 248 TB     │          32 │ citus       │ heap          │
└────────────┴──────────────────┴─────────────────────┴───────────────┴────────────┴─────────────┴─────────────┴───────────────┘

Time partitions view

Citus provides UDFs to manage partitions for the Timeseries Data use case. It also maintains a time_partitions view to inspect the partitions it manages.

Columns:

• parent_table the table which is partitioned

• partition_column the column on which the parent table is partitioned

• partition the name of a partition table

• from_value lower bound in time for rows in this partition

• to_value upper bound in time for rows in this partition

• access_method heap for row-based storage, and columnar for columnar storage

SELECT * FROM time_partitions;

┌────────────────────────┬──────────────────┬─────────────────────────────────────────┬─────────────────────┬─────────────────────┬───────────────┐
│      parent_table      │ partition_column │                partition                │     from_value      │      to_value       │ access_method │
├────────────────────────┼──────────────────┼─────────────────────────────────────────┼─────────────────────┼─────────────────────┼───────────────┤
│ github_columnar_events │ created_at       │ github_columnar_events_p2015_01_01_0000 │ 2015-01-01 00:00:00 │ 2015-01-01 02:00:00 │ columnar      │
│ github_columnar_events │ created_at       │ github_columnar_events_p2015_01_01_0200 │ 2015-01-01 02:00:00 │ 2015-01-01 04:00:00 │ columnar      │
│ github_columnar_events │ created_at       │ github_columnar_events_p2015_01_01_0400 │ 2015-01-01 04:00:00 │ 2015-01-01 06:00:00 │ columnar      │
│ github_columnar_events │ created_at       │ github_columnar_events_p2015_01_01_0600 │ 2015-01-01 06:00:00 │ 2015-01-01 08:00:00 │ heap          │
└────────────────────────┴──────────────────┴─────────────────────────────────────────┴─────────────────────┴─────────────────────┴───────────────┘

Co-location group table

The pg_dist_colocation table contains information about which tables’ shards should be placed together, or co-located. When two tables are in the same co-location group, Citus ensures shards with the same partition values will be placed on the same worker nodes. This enables join optimizations, certain distributed rollups, and foreign key support. Shard co-location is inferred when the shard counts, replication factors, and partition column types all match between two tables; however, a custom co-location group may be specified when creating a distributed table, if so desired.

SELECT * from pg_dist_colocation;
  colocationid | shardcount | replicationfactor | distributioncolumntype | distributioncolumncollation
 --------------+------------+-------------------+------------------------+-----------------------------
             2 |         32 |                 2 |                     20 |                           0
  (1 row)

Rebalancer strategy table

This table defines strategies that rebalance_table_shards can use to determine where to move shards.

A Citus installation ships with these strategies in the table:

SELECT * FROM pg_dist_rebalance_strategy;

-[ RECORD 1 ]------------------+---------------------------------
name                           | by_shard_count
default_strategy               | t
shard_cost_function            | citus_shard_cost_1
node_capacity_function         | citus_node_capacity_1
shard_allowed_on_node_function | citus_shard_allowed_on_node_true
default_threshold              | 0
minimum_threshold              | 0
improvement_threshold          | 0
-[ RECORD 2 ]------------------+---------------------------------
name                           | by_disk_size
default_strategy               | f
shard_cost_function            | citus_shard_cost_by_disk_size
node_capacity_function         | citus_node_capacity_1
shard_allowed_on_node_function | citus_shard_allowed_on_node_true
default_threshold              | 0.1
minimum_threshold              | 0.01
improvement_threshold          | 0.5

The default strategy, by_shard_count, assigns every shard the same cost. Its effect is to equalize the shard count across nodes. The other predefined strategy, by_disk_size, assigns a cost to each shard matching its disk size in bytes plus that of the shards that are colocated with it. The disk size is calculated using pg_total_relation_size, so it includes indices. This strategy attempts to achieve the same disk space on every node. Note the threshold of 0.1 – it prevents unnecessary shard movement caused by insigificant differences in disk space.

Creating custom rebalancer strategies

Here are examples of functions that can be used within new shard rebalancer strategies, and registered in the Rebalancer strategy table with the citus_add_rebalance_strategy function.

• Setting a node capacity exception by hostname pattern:

  -- example of node_capacity_function
  CREATE FUNCTION v2_node_double_capacity(nodeidarg int)
      RETURNS real AS $$
      SELECT
          (CASE WHEN nodename LIKE '%.v2.worker.citusdata.com' THEN 2.0::float4 ELSE 1.0::float4 END)
      FROM pg_dist_node where nodeid = nodeidarg
      $$ LANGUAGE sql;

• Rebalancing by number of queries that go to a shard, as measured by the Query statistics table:

  -- example of shard_cost_function
  CREATE FUNCTION cost_of_shard_by_number_of_queries(shardid bigint)
      RETURNS real AS $$
      SELECT coalesce(sum(calls)::real, 0.001) as shard_total_queries
      FROM citus_stat_statements
      WHERE partition_key is not null
          AND get_shard_id_for_distribution_column('tab', partition_key) = shardid;
  $$ LANGUAGE sql;

• Isolating a specific shard (10000) on a node (address ‘10.0.0.1’):

  -- example of shard_allowed_on_node_function
  CREATE FUNCTION isolate_shard_10000_on_10_0_0_1(shardid bigint, nodeidarg int)
      RETURNS boolean AS $$
      SELECT
          (CASE WHEN nodename = '10.0.0.1' THEN shardid = 10000 ELSE shardid != 10000 END)
      FROM pg_dist_node where nodeid = nodeidarg
      $$ LANGUAGE sql;
  -- The next two definitions are recommended in combination with the above function.
  -- This way the average utilization of nodes is not impacted by the isolated shard.
  CREATE FUNCTION no_capacity_for_10_0_0_1(nodeidarg int)
      RETURNS real AS $$
      SELECT
          (CASE WHEN nodename = '10.0.0.1' THEN 0 ELSE 1 END)::real
      FROM pg_dist_node where nodeid = nodeidarg
      $$ LANGUAGE sql;
  CREATE FUNCTION no_cost_for_10000(shardid bigint)
      RETURNS real AS $$
      SELECT
          (CASE WHEN shardid = 10000 THEN 0 ELSE 1 END)::real
      $$ LANGUAGE sql;

Query statistics table

Note

The citus_stat_statements view is a part of Citus Enterprise. Please contact us to obtain this functionality.

Citus provides citus_stat_statements for stats about how queries are being executed, and for whom. It’s analogous to (and can be joined with) the pg_stat_statements view in PostgreSQL which tracks statistics about query speed.

This view can trace queries to originating tenants in a multi-tenant application, which helps for deciding when to do Tenant Isolation.

-- create and populate distributed table
create table foo ( id int );
select create_distributed_table('foo', 'id');
insert into foo select generate_series(1,100);
-- enable stats
-- pg_stat_statements must be in shared_preload libraries
create extension pg_stat_statements;
select count(*) from foo;
select * from foo where id = 42;
select * from citus_stat_statements;

Results:

-[ RECORD 1 ]-+----------------------------------------------
queryid       | -909556869173432820
userid        | 10
dbid          | 13340
query         | insert into foo select generate_series($1,$2)
executor      | insert-select
partition_key |
calls         | 1
-[ RECORD 2 ]-+----------------------------------------------
queryid       | 3919808845681956665
userid        | 10
dbid          | 13340
query         | select count(*) from foo;
executor      | adaptive
partition_key |
calls         | 1
-[ RECORD 3 ]-+----------------------------------------------
queryid       | 5351346905785208738
userid        | 10
dbid          | 13340
query         | select * from foo where id = $1
executor      | adaptive
partition_key | 42
calls         | 1

Caveats:

• The stats data is not replicated, and won’t survive database crashes or failover

• Tracks a limited number of queries, set by the pg_stat_statements.max GUC (default 5000)

• To truncate the table, use the citus_stat_statements_reset() function

Distributed Query Activity

In some situations, queries might get blocked on row-level locks on one of the shards on a worker node. If that happens then those queries would not show up in pg_locks on the Citus coordinator node.

Citus provides special views to watch queries and locks throughout the cluster, including shard-specific queries used internally to build results for distributed queries.

• citus_dist_stat_activity: shows the distributed queries that are executing on all nodes. A superset of pg_stat_activity, usable wherever the latter is.

• citus_worker_stat_activity: shows queries on workers, including fragment queries against individual shards.

• citus_lock_waits: Blocked queries throughout the cluster.

The first two views include all columns of pg_stat_activity plus the host/port of the worker that initiated the query and the host/port of the coordinator node of the cluster.

For example, consider counting the rows in a distributed table:

-- run from worker on localhost:9701
SELECT count(*) FROM users_table;

We can see the query appear in citus_dist_stat_activity:

SELECT * FROM citus_dist_stat_activity;
-[ RECORD 1 ]----------+----------------------------------
query_hostname         | localhost
query_hostport         | 9701
master_query_host_name | localhost
master_query_host_port | 9701
transaction_number     | 1
transaction_stamp      | 2018-10-05 13:27:20.691907+03
datid                  | 12630
datname                | postgres
pid                    | 23723
usesysid               | 10
usename                | citus
application_name       | psql
client_addr            |
client_hostname        |
client_port            | -1
backend_start          | 2018-10-05 13:27:14.419905+03
xact_start             | 2018-10-05 13:27:16.362887+03
query_start            | 2018-10-05 13:27:20.682452+03
state_change           | 2018-10-05 13:27:20.896546+03
wait_event_type        | Client
wait_event             | ClientRead
state                  | idle in transaction
backend_xid            |
backend_xmin           |
query                  | SELECT count(*) FROM users_table;
backend_type           | client backend

This query requires information from all shards. Some of the information is in shard users_table_102038 which happens to be stored in localhost:9700. We can see a query accessing the shard by looking at the citus_worker_stat_activity view:

SELECT * FROM citus_worker_stat_activity;
-[ RECORD 1 ]----------+-----------------------------------------------------------------------------------------
query_hostname         | localhost
query_hostport         | 9700
master_query_host_name | localhost
master_query_host_port | 9701
transaction_number     | 1
transaction_stamp      | 2018-10-05 13:27:20.691907+03
datid                  | 12630
datname                | postgres
pid                    | 23781
usesysid               | 10
usename                | citus
application_name       | citus
client_addr            | ::1
client_hostname        |
client_port            | 51773
backend_start          | 2018-10-05 13:27:20.75839+03
xact_start             | 2018-10-05 13:27:20.84112+03
query_start            | 2018-10-05 13:27:20.867446+03
state_change           | 2018-10-05 13:27:20.869889+03
wait_event_type        | Client
wait_event             | ClientRead
state                  | idle in transaction
backend_xid            |
backend_xmin           |
query                  | COPY (SELECT count(*) AS count FROM users_table_102038 users_table WHERE true) TO STDOUT
backend_type           | client backend

The query field shows data being copied out of the shard to be counted.

Note

If a router query (e.g. single-tenant in a multi-tenant application, SELECT * FROM table WHERE tenant_id = X) is executed without a transaction block, then citus_query_host_name and citus_query_host_port columns will be NULL in citus_worker_stat_activity.

Here are examples of useful queries you can build using citus_worker_stat_activity:

-- active queries' wait events on a certain node
SELECT query, wait_event_type, wait_event
  FROM citus_worker_stat_activity
 WHERE query_hostname = 'xxxx' and state='active';
-- active queries' top wait events
SELECT wait_event, wait_event_type, count(*)
  FROM citus_worker_stat_activity
 WHERE state='active'
 GROUP BY wait_event, wait_event_type
 ORDER BY count(*) desc;
-- total internal connections generated per node by Citus
SELECT query_hostname, count(*)
  FROM citus_worker_stat_activity
 GROUP BY query_hostname;
-- total internal active connections generated per node by Citus
SELECT query_hostname, count(*)
  FROM citus_worker_stat_activity
 WHERE state='active'
 GROUP BY query_hostname;

The next view is citus_lock_waits. To see how it works, we can generate a locking situation manually. First we’ll set up a test table from the coordinator:

CREATE TABLE numbers AS
  SELECT i, 0 AS j FROM generate_series(1,10) AS i;
SELECT create_distributed_table('numbers', 'i');

Then, using two sessions on the coordinator, we can run this sequence of statements:

-- session 1                           -- session 2
-------------------------------------  -------------------------------------
BEGIN;
UPDATE numbers SET j = 2 WHERE i = 1;
                                       BEGIN;
                                       UPDATE numbers SET j = 3 WHERE i = 1;
                                       -- (this blocks)

The citus_lock_waits view shows the situation.

SELECT * FROM citus_lock_waits;
-[ RECORD 1 ]-------------------------+----------------------------------------
waiting_pid                           | 88624
blocking_pid                          | 88615
blocked_statement                     | UPDATE numbers SET j = 3 WHERE i = 1;
current_statement_in_blocking_process | UPDATE numbers SET j = 2 WHERE i = 1;
waiting_node_id                       | 0
blocking_node_id                      | 0
waiting_node_name                     | coordinator_host
blocking_node_name                    | coordinator_host
waiting_node_port                     | 5432
blocking_node_port                    | 5432

In this example the queries originated on the coordinator, but the view can also list locks between queries originating on workers.

Tables on all Nodes

Citus has other informational tables and views which are accessible on all nodes, not just the coordinator.

Connection Credentials Table

Note

This table is a part of Citus Enterprise Edition. Please contact us to obtain this functionality.

The pg_dist_authinfo table holds authentication parameters used by Citus nodes to connect to one another.

Upon beginning a connection, a node consults the table to see whether a row with the destination nodeid and desired rolename exists. If so, the node includes the corresponding authinfo string in its libpq connection. A common example is to store a password, like 'password=abc123', but you can review the full list of possibilities.

The parameters in authinfo are space-separated, in the form key=val. To write an empty value, or a value containing spaces, surround it with single quotes, e.g., keyword='a value'. Single quotes and backslashes within the value must be escaped with a backslash, i.e., \' and \\.

The nodeid column can also take the special values 0 and -1, which mean all nodes or loopback connections, respectively. If, for a given node, both specific and all-node rules exist, the specific rule has precedence.

SELECT * FROM pg_dist_authinfo;
 nodeid | rolename | authinfo
--------+----------+-----------------
    123 | jdoe     | password=abc123
(1 row)

Connection Pooling Credentials

Note

This table is a part of Citus Enterprise Edition. Please contact us to obtain this functionality.

If you want to use a connection pooler to connect to a node, you can specify the pooler options using pg_dist_poolinfo. This metadata table holds the host, port and database name for Citus to use when connecting to a node through a pooler.

If pool information is present, Citus will try to use these values instead of setting up a direct connection. The pg_dist_poolinfo information in this case supersedes pg_dist_node.

Note

In some situations Citus ignores the settings in pg_dist_poolinfo. For instance Shard rebalancing is not compatible with connection poolers such as pgbouncer. In these scenarios Citus will use a direct connection.

-- how to connect to node 1 (as identified in pg_dist_node)
INSERT INTO pg_dist_poolinfo (nodeid, poolinfo)
     VALUES (1, 'host=127.0.0.1 port=5433');