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51.11. pg_class

The catalog pg_class describes tables and other objects that have columns or are otherwise similar to a table. This includes indexes (but see also pg_index), sequences (but see also pg_sequence), views, materialized views, composite types, and TOAST tables; see relkind. Below, when we mean all of these kinds of objects we speak of relations. Not all of pg_class's columns are meaningful for all relation kinds.

Table 51.11. pg_class Columns

Name Type References Description
oid oid   Row identifier
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 a table or an index, the access method used (heap, 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 live 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.
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, t = TOAST table, v = view, m = materialized view, c = composite type, f = foreign table, p = partitioned table, I = partitioned index
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
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 or index has (or once had) any inheritance children
relrowsecurity bool   True if table has row level security enabled; see pg_policy catalog
relforcerowsecurity bool   True if row level security (when enabled) will also apply to table owner; see pg_policy catalog
relispopulated bool   True if relation is populated (this is true for all relations other than some materialized views)
relreplident char   Columns used to form replica identity for rows: d = default (primary key, if any), n = nothing, f = all columns, i = index with indisreplident set (same as nothing if the index used has been dropped)
relispartition bool   True if table or index is a partition
relrewrite oid pg_class.oid For new relations being written during a DDL operation that requires a table rewrite, this contains the OID of the original relation; otherwise 0. That state is only visible internally; this field should never contain anything other than 0 for a user-visible relation.
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_xact to be shrunk. Zero (InvalidTransactionId) if the relation is not a table.
relminmxid xid   All multixact IDs before this one have been replaced by a transaction ID in this table. This is used to track whether the table needs to be vacuumed in order to prevent multixact ID wraparound or to allow pg_multixact to be shrunk. Zero (InvalidMultiXactId) if the relation is not a table.
relacl aclitem[]   Access privileges; see Section 5.7 for details
reloptions text[]   Access-method-specific options, as keyword=value strings
relpartbound pg_node_tree   If table is a partition (see relispartition), internal representation of the partition bound

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.

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