52.7. 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 52.7. pg_attribute Columns
| 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 ctid, 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. | |
atthasdef | bool | This column has a default expression or generation expression, in which case there will be a corresponding entry in the pg_attrdef catalog that actually defines the expression. (Check attgenerated to determine whether this is a default or a generation expression.) | |
atthasmissing | bool | This column has a value which is used where the column is entirely missing from the row, as happens when a column is added with a non-volatile DEFAULT value after the row is created. The actual value used is stored in the attmissingval column. | |
attidentity | char | If a zero byte (''), then not an identity column. Otherwise, a = generated always, d = generated by default. | |
attgenerated | char | If a zero byte (''), then not a generated column. Otherwise, s = stored. (Other values might be added in the future.) | |
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 | |
attfdwoptions | text[] | Attribute-level foreign data wrapper options, as “keyword=value” strings | |
attmissingval | anyarray | This column has a one element array containing the value used when the column is entirely missing from the row, as happens when the column is added with a non-volatile DEFAULT value after the row is created. The value is only used when atthasmissing is true. If there is no value the column is null. |
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.