pg_class

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

column	type	references	description
oid	oid		Row identifier
relname	name		Name of the table, index, view.
relnamespace	oid	pg_namespace.oid	The object identifier of the namespace (schema) that contains this relation
reltype	oid	pg_type.oid	The object identifier 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 object identifier 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.)
relfilenode	bigint		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	real		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	The object identifier 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	boolean		True if this is a table and it has (or recently had) any indexes.
relisshared	boolean		True if this table is shared across all databases in the system. Only certain system catalog tables (such as pg_database) are shared.
relpersistence	char		The type of object persistence: p = heap or append-optimized permanent table, u = unlogged temporary table, t = temporary table.
relkind	char		The type of object r = heap or append-optimized 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, u = uncatalogued temporary heap table, o = internal append-optimized segment files and EOFs, b = append-only block directory, M = append-only visibility map.
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	boolean		True if table has (or once had) rules; see pg_rewrite catalog.
relhastriggers	boolean		True if table has (or once had) triggers.
relhassubclass	boolean		True if table has (or once had) any inheritance children.
relrowsecurity	boolean		True if table has row level security enabled; see pg_policy catalog.
relforcerowsecurity	boolean		True if row level security (when enabled) will also apply to the table owner; see pg_policy catalog.
relispopulated	boolean		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	boolean		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 object identifier 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. The value is 0 (InvalidTransactionId) if the relation is not a table or if the table does not require vacuuming to prevent transaction ID wraparound. The table still might require vacuuming to reclaim disk space.
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 assigned by GRANT and REVOKE.
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 might 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.