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45.11. pg_class

The catalog pg_class catalogs tables and most everything else that has columns or is otherwise similar to a table. This includes indexes (but see also pg_index), sequences, views, composite types, and TOAST tables; see relkind. Below, when we mean all of these kinds of objects we speak of "relations". Not all columns are meaningful for all relation types.

Table 45-11. pg_class Columns

Name Type References Description
relname name   Name of the table, index, view, etc.
relnamespace oid pg_namespace.oid The OID of the namespace that contains this relation
reltype oid pg_type.oid The OID of the data type that corresponds to this table's row type, if any (zero for indexes, which have no pg_type entry)
reloftype oid pg_type.oid For typed tables, the OID of the underlying composite type, zero for all other relations
relowner oid pg_authid.oid Owner of the relation
relam oid pg_am.oid If this is an index, the access method used (B-tree, hash, etc.)
relfilenode oid   Name of the on-disk file of this relation; zero means this is a "mapped" relation whose disk file name is determined by low-level state
reltablespace oid pg_tablespace.oid The tablespace in which this relation is stored. If zero, the database's default tablespace is implied. (Not meaningful if the relation has no on-disk file.)
relpages int4   Size of the on-disk representation of this table in pages (of size BLCKSZ). This is only an estimate used by the planner. It is updated by VACUUM, ANALYZE, and a few DDL commands such as CREATE INDEX.
reltuples float4   Number of rows in the table. This is only an estimate used by the planner. It is updated by VACUUM, ANALYZE, and a few DDL commands such as CREATE INDEX.
relallvisible int4   Number of pages that are marked all-visible in the table's visibility map. This is only an estimate used by the planner. It is updated by VACUUM, ANALYZE, and a few DDL commands such as CREATE INDEX.
reltoastrelid oid pg_class.oid OID of the TOAST table associated with this table, 0 if none. The TOAST table stores large attributes "out of line" in a secondary table.
reltoastidxid oid pg_class.oid For a TOAST table, the OID of its index. 0 if not a TOAST table.
relhasindex bool   True if this is a table and it has (or recently had) any indexes
relisshared bool   True if this table is shared across all databases in the cluster. Only certain system catalogs (such as pg_database) are shared.
relpersistence char   p = permanent table, u = unlogged table, t = temporary table
relkind char   r = ordinary table, i = index, S = sequence, v = view, c = composite type, t = TOAST table, f = foreign table
relnatts int2   Number of user columns in the relation (system columns not counted). There must be this many corresponding entries in pg_attribute. See also pg_attribute.attnum.
relchecks int2   Number of CHECK constraints on the table; see pg_constraint catalog
relhasoids bool   True if we generate an OID for each row of the relation
relhaspkey bool   True if the table has (or once had) a primary key
relhasrules bool   True if table has (or once had) rules; see pg_rewrite catalog
relhastriggers bool   True if table has (or once had) triggers; see pg_trigger catalog
relhassubclass bool   True if table has (or once had) any inheritance children
relfrozenxid xid   All transaction IDs before this one have been replaced with a permanent ("frozen") transaction ID in this table. This 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. Zero (InvalidTransactionId) if the relation is not a table.
relacl aclitem[]   Access privileges; see GRANT and REVOKE for details
reloptions text[]   Access-method-specific options, as "keyword=value" strings

Several of the Boolean flags in pg_class are maintained lazily: they are guaranteed to be true if that's the correct state, but may not be reset to false immediately when the condition is no longer true. For example, relhasindex is set by CREATE INDEX, but it is never cleared by DROP INDEX. Instead, VACUUM clears relhasindex if it finds the table has no indexes. This arrangement avoids race conditions and improves concurrency.