Metadata Tables Reference

Citus divides each distributed table into multiple logical shards based on the distribution column. The master 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 master 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.

Name Type Description
logicalrelid regclass
Distributed table to which this row corresponds. This value references
the relfilenode column in the pg_class system catalog table.
partmethod char
The method used for partitioning / distribution. The values of this
column corresponding to different distribution methods are :-
append: ‘a’
hash: ‘h’
partkey text
Detailed information about the distribution column including column
number, type and other relevant information.
colocationid integer
Co-location group to which this table belongs. Tables in the same group
allow co-located joins and distributed rollups among other
optimizations. This value references the colocationid column in the
pg_dist_colocation table.
repmodel char
The method used for data replication. The values of this column
corresponding to different replication methods are :-
citus statement-based replication: ‘c’
postgresql streaming replication: ‘s’
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.

Name Type Description
logicalrelid regclass
Distributed table to which this shard belongs. This value references the
relfilenode column in the pg_class system catalog table.
shardid bigint
Globally unique identifier assigned to this shard.
shardstorage char
Type of storage used for this shard. Different storage types are
discussed in the table below.
shardminvalue text
For append distributed tables, minimum value of the distribution column
in this shard (inclusive).
For hash distributed tables, minimum hash token value assigned to that
shard (inclusive).
shardmaxvalue text
For append distributed tables, maximum value of the distribution column
in this shard (inclusive).
For hash distributed tables, maximum hash token value assigned to that
shard (inclusive).
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.

Storage Type Shardstorage value Description
TABLE ‘t’
Indicates that shard stores data belonging to a regular
distributed table.
COLUMNAR ‘c’
Indicates that shard stores columnar data. (Used by
distributed cstore_fdw tables)
FOREIGN ‘f’
Indicates that shard stores foreign data. (Used by
distributed file_fdw tables)

Shard placement table

The pg_dist_shard_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.

Name Type Description
shardid bigint
Shard identifier associated with this placement. This values references
the shardid column in the pg_dist_shard catalog table.
shardstate int
Describes the state of this placement. Different shard states are
discussed in the section below.
shardlength bigint
For append distributed tables, the size of the shard placement on the
worker node in bytes.
For hash distributed tables, zero.
nodename text
Host name or IP address of the worker node PostgreSQL server hosting
this shard placement.
nodeport int
Port number on which the worker node PostgreSQL server hosting this
shard placement is listening.
placementid bigint
Unique auto-generated identifier for each individual 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

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_shard_placement table is used to store the state of shard placements. A brief overview of different shard placement states and their representation is below.

State name Shardstate value Description
FINALIZED 1
This is the state new shards are created in. Shard placements
in this state are considered up-to-date and are used in query
planning and execution.
INACTIVE 3
Shard placements in this state are considered inactive due to
being out-of-sync with other replicas of the same shard. This
can occur when an append, modification (INSERT, UPDATE or
DELETE ) or a DDL operation fails for this placement. The query
planner will ignore placements in this state during planning and
execution. Users can synchronize the data in these shards with
a finalized replica as a background activity.
TO_DELETE 4
If Citus attempts to drop a shard placement in response to a
master_apply_delete_command call and fails, the placement is
moved to this state. Users can then delete these shards as a
subsequent background activity.

Worker node table

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

Name Type Description
nodeid int
Auto-generated identifier for an individual node.
groupid int
Identifier used to denote a group of one primary server and zero or more
secondary servers, when the streaming replication model is used. By
default it is the same as the nodeid.
nodename text
Host Name or IP Address of the PostgreSQL worker node.
nodeport int
Port number on which the PostgreSQL worker node is listening.
noderack text
(Optional) Rack placement information for the worker node.
hasmetadata boolean
Reserved for internal use.
SELECT * from pg_dist_node;
 nodeid | groupid | nodename  | nodeport | noderack | hasmetadata
--------+---------+-----------+----------+----------+-------------
      1 |       1 | localhost |    12345 | default  | f
      2 |       2 | localhost |    12346 | default  | f
      3 |       3 | localhost |    12347 | default  | f
(3 rows)

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.

Name Type Description
colocationid int
Unique identifier for the co-location group this row corresponds to.
shardcount int
Shard count for all tables in this co-location group
replicationfactor int
Replication factor for all tables in this co-location group.
distributioncolumntype oid
The type of the distribution column for all tables in this
co-location group.
SELECT * from pg_dist_colocation;
  colocationid | shardcount | replicationfactor | distributioncolumntype
 --------------+------------+-------------------+------------------------
             2 |         32 |                 2 |                     20
  (1 row)