51.7. pg_attribute

51.7. `pg_attribute`
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51.7. `pg_attribute`

The catalog pg_attribute 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 indeed all objects that have pg_class entries.)

The term attribute is equivalent to column and is used for historical reasons.

Table 51.7. pg_attribute Columns

Name	Type	References	Description
`attrelid`	`oid`	`pg_class.oid`	The table this column belongs to
`attname`	`name`		The column name
`atttypid`	`oid`	`pg_type.oid`	The data type of this column
`attstattarget`	`int4`		`attstattarget` 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, `attstattarget` is both the target number of “most common values” to collect, and the target number of histogram bins to create.
`attlen`	`int2`		A copy of `pg_type.typlen` of this column's type
`attnum`	`int2`		The number of the column. Ordinary columns are numbered from 1 up. System columns, such as `ctid`, have (arbitrary) negative numbers.
`attndims`	`int4`		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's an array”.)
`attcacheoff`	`int4`		Always -1 in storage, but when loaded into a row descriptor in memory this might be updated to cache the offset of the attribute within the row
`atttypmod`	`int4`		`atttypmod` 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 `atttypmod`.
`attbyval`	`bool`		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`	`bool`		This represents a not-null constraint.
`atthasdef`	`bool`		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 expression. (Check `attgenerated` to determine whether this is a default or a generation expression.)
`atthasmissing`	`bool`		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`	`bool`		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`	`bool`		This column is defined locally in the relation. Note that a column can be locally defined and inherited simultaneously.
`attinhcount`	`int4`		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 column 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.

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 cope with 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.