PostgreSQL 9.3.6 Documentation
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# 47.11. `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, 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 columns are meaningful for all relation types.

Table 47-11. `pg_class` Columns

Name | Type | References | Description
--- | --- | --- | ---
`oid` | `oid` |  | Row identifier (hidden attribute; must be explicitly selected)
`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, `m` = materialized 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
`relispopulated` | `bool` |  | True if relation is populated (this is true for all relations other than some materialized views)
`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.
`relminmxid` | `xid` |  | All multitransaction 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 multitransaction ID 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.

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