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45.7. pg_attribute
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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 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attisdropped bool   This column has been dropped and is no longer valid.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.
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

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。

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