历史版本。 PostgreSQL 9.1 已结束支持。 2016-10-27. 请参阅 当前版本手册.
45.7. pg_attribute #
目录pg_attribute存储关于表列的信息。数据库中每个表的每一列，在pg_attribute中都恰好有一行。（其中也包括索引的属性项，实际上，凡是在pg_class中有项的对象，都有对应的属性项。）
术语属性等同于列，这里使用它只是出于历史原因。
表 45.7. pg_attribute 列
| Name | Type | References | Description |
|---|---|---|---|
attrelid | oid | | The table this column belongs to |
attname | name | The column name | |
atttypid | 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attisdropped | bool | This column has been dropped and is no longer valid.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. | |
attcollation | 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 |
在被删除列的 pg_attribute 条目中，atttypid 被重置为零，但 attlen 以及其他从 pg_type 复制的字段仍然有效。这种安排用于应对被删除列的数据类型后来也被删除、因而不再有相应 pg_type 行的情况。attlen 和其他字段可用于解释表中一行的内容。