51.11 pg_class
版本:11
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 (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 columns are meaningful for all relation types.
Table 51.11. pg_class
Columns
pg_class
ColumnsColumn Type Description |
Row identifier |
Name of the table, index, view, etc. |
The OID of the namespace that contains this relation |
The OID of the data type that corresponds to this table's row type, if any (zero for indexes, which have no |
For typed tables, the OID of the underlying composite type, zero for all other relations |
Owner of the relation |
If this is a table or an index, the access method used (heap, B-tree, hash, etc.) |
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 |
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.) |
Size of the on-disk representation of this table in pages (of size |
Number of live rows in the table. This is only an estimate used by the planner. It is updated by |
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 |
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. |
True if this is a table and it has (or recently had) any indexes |
True if this table is shared across all databases in the cluster. Only certain system catalogs (such as |
|
|
Number of user columns in the relation (system columns not counted). There must be this many corresponding entries in |
|
|
|
True if table or index has (or once had) any inheritance children |
|
|
True if relation is populated (this is true for all relations other than some materialized views) |
Columns used to form “replica identity” for rows: |
True if table or index is a partition |
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. |
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 |
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 |
|
Access-method-specific options, as “keyword=value” strings |
If table is a partition (see |
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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