PostgreSQL 8.4.4 Documentation
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# 44.7. `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 44-7. `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 `oid`, 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. It is possible to change this column to enable or disable the constraint
`atthasdef` | `bool` |  | This column has a default value, in which case there will be a corresponding entry in the `pg_attrdef` catalog that actually defines the value
`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
`attacl` | `aclitem[]` |  | Column-level access privileges, if any have been granted specifically on this 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 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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