pg_attribute

PostgreSQL 7.4.18 Documentation
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# 43.7. `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 other objects. See `pg_class`.)

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

Table 43-7. `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 may 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.
`attisset` | `bool` |  | If true, this attribute is a set. In that case, what is really stored in the attribute is the OID of a row in the `pg_proc` catalog. The `pg_proc` row contains the query string that defines this set, i.e., the query to run to get the set. So the `atttypid` (see above) refers to the type returned by this query, but the actual length of this attribute is the length (size) of an `oid`. --- At least this is the theory. All this is probably quite broken these days.
`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 may 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.

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