# pg_attribute

The `pg_attribute` table stores information about table columns. There will be exactly one `pg_attribute` row for every column in every table in the database. (There will also be attribute entries for indexes, and all objects that have `pg_class` entries.) The term attribute is equivalent to column.

In a dropped column's `pg_attribute` entry, `atttypid` is reset to zero, but attlen and the other fields copied from `pg_type` are still valid. This arrangement is needed to address the situation where the dropped column's data type was later dropped, and so there is no `pg_type` row anymore. `attlen` and the other fields can be used to interpret the contents of a row of the table

| column          | type     | references            | description                                                  |
| :-------------- | :------- | :-------------------- | :----------------------------------------------------------- |
|`attrelid`	|oid|	[pg_class](./pg_class.md).oid|	The table this column belongs to.
|`attname`	|name|		|The column name.
|`atttypid`	|oid	|[pg_type](./pg_type.md).oid|	The data type of this column.
|`attstattarget`	|integer|		|Controls the level of detail of statistics accumulated for this column by `ANALYZE`. A zero value indicates that no statistics should be collected. A negative value says to use the system default statistics target. The exact meaning of positive values is data type-dependent. For scalar data types, it is both the target number of "most common values" to collect, and the target number of histogram bins to create.
|`attlen`	|smallint|		|A copy of `pg_type.typlen` of this column's type.
|`attnum`	|smallint	|	|The number of the column. Ordinary columns are numbered from 1 up. System columns, such as ctid, have |(arbitrary) negative numbers.
|`attndims`	|integer|	|	Number of dimensions, if the column is an array type; otherwise 0. (Presently, the number of dimensions of an array is not enforced, so any nonzero value effectively means it is an array.)
|`attcacheoff`|	integer||		Always -1 in storage, but when loaded into a row descriptor in memory this may be updated to cache the offset of the attribute within the row.
|`atttypmod`	|integer	||	Records type-specific data supplied at table creation time (for example, the maximum length of a `varchar` column). It is passed to type-specific input functions and length coercion functions. The value will generally be -1 for types that do not need it.
|`attbyval`	|boolean|	|	A copy of `pg_type.typbyval` of this column's type.
|`attstorage`	|char	|	|Normally a copy of `pg_type.typstorage` of this column's type. For TOAST-able data types, this can be altered after column creation to control storage policy.
|`attalign`	|char	|	|A copy of `pg_type.typalign` of this column's type.
|`attnotnull`	|boolean|	|	This represents a not-null constraint.
|`atthasdef`	|boolean	|	|This column has a default expression or generation expression, in which case there will be a corresponding entry in the pg_attrdef catalog that actually defines the value. (Check `attgenerated` to determine whether this is a default or a generation expression.)
|`atthasmissing`|	boolean|	|	This column has a value which is used where the column is entirely missing from the row, as happens when a column is added with a non-volatile `DEFAULT` value after the row is created. The actual value used is stored in the `attmissingval` column.
|`attidentity`|	char|		|If a zero byte (''), then not an identity column. Otherwise, a = generated always, d = generated by default.
|`attgenerated`|	char	|	|If a zero byte (''), then not a generated column. Otherwise, s = stored. (Other values might be added in the future.)
|`attisdropped`|	boolean	|	|This column has been dropped and is no longer valid. A dropped column is still physically present in the table, but is ignored by the parser and so cannot be accessed via SQL.
|`attislocal`	|boolean	|	|This column is defined locally in the relation. Note that a column may be locally defined and inherited simultaneously.
|`attinhcount`|	integer	|	|The number of direct ancestors this column has. A column with a nonzero number of ancestors cannot be dropped nor renamed.
|`attcollation`|	oid	|pg_collation.oid|	The defined collation of the column, or zero if the is not of a collatable data type.
|`attacl`	|aclitem[]		||Column-level access privileges, if any have been granted specifically on this column.
|`attoptions`	|text[]		||Attribute-level options, as "keyword=value" strings.
|`attfdwoptions`|	text[]	||	Attribute-level foreign data wrapper options, as "keyword=value" strings.
|`attmissingval`|	anyarray||		This column has a one element array containing the value used when the column is entirely missing from the row, as happens when the column is added with a non-volatile `DEFAULT` value after the row is created. The value is only used when `atthasmissing` is true. If there is no value the column is null.