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 indeed all objects that have pg_class entries.)

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
`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

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.