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PG_CLASS

PG_CLASS records database objects and their relations.

Table 1 PG_CLASS columns

Column

Type

Description

OID

OID

Row identifier (hidden attribute; must be explicitly selected)

relname

Name

Name of an object, such as a table, index, or view

relnamespace

OID

OID of the namespace that contains the relationship

reltype

OID

Data type that corresponds to this table's row type (the index is 0 because the index does not have pg_type record)

reloftype

OID

OID is of composite type. 0 indicates other types.

relowner

OID

Owner of the relationship

relam

OID

Specifies the access method used, such as B-tree and hash, if this is an index

relfilenode

OID

Name of the on-disk file of this relationship. If such file does not exist, the value is 0.

reltablespace

OID

Tablespace in which this relationship is stored. If its value is 0, the default tablespace in this database is used. This column is meaningless if the relationship has no on-disk file.

relpages

Double precision

Size of the on-disk representation of this table in pages (of size BLCKSZ). This is only an estimate used by the optimizer.

reltuples

Double precision

Number of rows in the table. This is only an estimate used by the optimizer.

relallvisible

Integer

Number of pages marked as all visible in the table. This column is used by the optimizer for optimizing SQL execution. It is updated by VACUUM, ANALYZE, and a few DDL statements such as CREATE INDEX.

reltoastrelid

OID

OID of the TOAST table associated with this table. The OID is 0 if no TOAST table exists.

The TOAST table stores large columns "offline" in a secondary table.

reltoastidxid

OID

OID of the index for a TOAST table. The OID is 0 for a table other than a TOAST table.

reldeltarelid

OID

OID of a Delta table

Delta tables belong to column-store tables. They store long tail data generated during data import.

reldeltaidx

OID

OID of the index for a Delta table

relcudescrelid

OID

OID of a CU description table

CU description tables (Desc tables) belong to column-store tables. They control whether storage data in the HDFS table directory is visible.

relcudescidx

OID

OID of the index for a CU description table

relhasindex

boolean

Its value is true if this column is a table and has (or recently had) at least one index.

It is set by CREATE INDEX but is not immediately cleared by DROP INDEX. If the VACUUM process detects that a table has no index, it clears the relhasindex column and sets the value to false.

relisshared

boolean

Its value is true if the table is shared across all databases in the cluster. Only certain system catalogs (such as pg_database) are shared.

relpersistence

Char
  • p indicates a permanent table.
  • u indicates a non-log table.
  • t indicates a local temporary table.
  • g indicates a global temporary table.

relkind

Char
  • r indicates an ordinary table.
  • i indicates an index.
  • S indicates a sequence.
  • v indicates a view.
  • c indicates the composite type.
  • t indicates a TOAST table.
  • f indicates a foreign table.
  • m indicates a materialized view.

relnatts

Smallint

Number of user columns in the relationship (excluding system columns) pg_attribute has the same number of rows corresponding to the user columns.

relchecks

Smallint

Number of constraints on a table. For details, see PG_CONSTRAINT.

relhasoids

boolean

Its value is true if an OID is generated for each row of the relationship.

relhaspkey

boolean

Its value is true if the table has (or once had) a primary key.

relhasrules

boolean

Its value is true if the table has rules. See table PG_REWRITE to check whether it has rules.

relhastriggers

boolean

Its value is true if the table has (or once had) triggers. For details, see PG_TRIGGER.

relhassubclass

boolean

Its value is true if the table has (or once had) any inheritance child table.

relcmprs

tinyint

Whether the compression feature is enabled for the table. Note that only batch insertion triggers compression so ordinary CRUD does not trigger compression.

  • 0 indicates other tables that do not support compression (primarily system tables, on which the compression attribute cannot be modified).
  • 1 indicates that the compression feature of the table data is NOCOMPRESS or has no specified keyword.
  • 2 indicates that the compression feature of the table data is COMPRESS.

relhasclusterkey

boolean

Whether the local cluster storage is used

relrowmovement

boolean

Whether the row migration is allowed when the partitioned table is updated

  • true indicates that the row migration is allowed.
  • false indicates that the row migration is not allowed.

parttype

Char

Whether the table or index has the property of a partitioned table

  • p indicates that the table or index has the property of a partitioned table.
  • n indicates that the table or index does not have the property of a partitioned table.
  • v indicates that the table is the value partitioned table in the HDFS.

relfrozenxid

xid32

All transaction IDs before this one have been replaced with a permanent ("frozen") transaction ID in this table. This column is used to track whether the table needs to be vacuumed in order to prevent transaction ID wraparound (or to allow pg_clog to be shrunk). The value is 0 (InvalidTransactionId) if the relationship is not a table.

To ensure forward compatibility, this column is reserved. The relfrozenxid64 column is added to record the information.

relacl

aclitem[]

Access permissions

The command output of the query is as follows:

1
 
rolename=xxxx/yyyy  --Assigning privileges to a role

1
 
=xxxx/yyyy --Assigning the permission to public

xxxx indicates the assigned privileges, and yyyy indicates the roles that are assigned to the privileges. For details about permission descriptions, see Table 2.

reloptions

text[]

Access-method-specific options, as "keyword=value" strings

relfrozenxid64

Xid

All transaction IDs before this one have been replaced with a permanent ("frozen") transaction ID in this table. This column is used to track whether the table needs to be vacuumed in order to prevent transaction ID wraparound (or to allow pg_clog to be shrunk). The value is 0 (InvalidTransactionId) if the relationship is not a table.

Database object permissions are controlled using access control lists (ACLs). DWS uses aclitem to indicate the permissions on a specific database object.

  • For databases and schemas, aclitem is stored in pg_database.datacl and pg_namespace.nspacl.
  • For other database objects such as tables and views, aclitem is stored in pg_class.relacl.
  • For column-level permissions, aclitem is stored in pg_attribute.attacl.

For example, test_user=a*r/test_user1 indicates that the test_user user has the INSERT and SELECT permissions on the current database object. The INSERT permission with WITH GRANT OPTION,means that other users can be granted the permission. /test_user1 indicates that the aclitem permission is granted by test_user1.

The following table lists all database object permissions and their abbreviations in DWS.

Table 2 ACL permission parameters

Permission

Abbreviation

Applicable Object Type

SELECT (read)

r

LARGE OBJECT, SEQUENCE, TABLE (and table-like objects), and table column

UPDATE (write)

w

LARGE OBJECT, SEQUENCE, TABLE, and table column

INSERT (insert)

a

TABLE and table column

DELETE

d

TABLE

TRUNCATE

D

TABLE

REFERENCES

x

TABLE and table column

TRIGGER

t

TABLE

EXECUTE

X

FUNCTION and PROCEDURE

USAGE

U

DOMAIN, FOREIGN DATA WRAPPER, FOREIGN SERVER, LANGUAGE, SCHEMA, SEQUENCE, and TYPE

CREATE

C

DATABASE, SCHEMA, and TABLESPACE

CONNECT

c

DATABASE

TEMPORARY

T

DATABASE

ANALYZE|ANALYSE

A

TABLE

ALTER

L

TABLE

DROP

P

TABLE

VACUUM

v

TABLE

ALL PRIVILEGES

arwdDxtA and vLP

TABLE

Authorization options for preceding permissions

*

-

Examples

View the OID and relfilenode of a table.

1
 
SELECT oid,relname,relfilenode FROM pg_class WHERE relname = 'table_name';

Count row-store tables.

1
 
SELECT 'row count:'||count(1) as point FROM pg_class WHERE relkind = 'r' and oid > 16384 and reloptions::text not like '%column%' and reloptions::text not like '%internal_mask%';

Count column-store tables.

1
 
SELECT 'column count:'||count(1) as point FROM pg_class WHERE relkind = 'r' and oid > 16384 and reloptions::text like '%column%';

Query the comments of all tables in the database.

1
 
SELECT relname as tablename,obj_description(relfilenode,'pg_class') as comment FROM pg_class;

Query the permission of a specified table in the database.

1
 
SELECT relacl FROM pg_class WHERE relname = 'table_name';
Parent Topic: System Catalogs