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.

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