The pg_attribute
table 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 all objects that have pg_class
entries.) The term attribute is equivalent to column.
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 address 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
column | 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 |
integer | 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, it is both the target number of "most common values" to collect, and the target number of histogram bins to create. | |
attlen |
smallint | A copy of pg_type.typlen of this column's type. |
|
attnum |
smallint | The number of the column. Ordinary columns are numbered from 1 up. System columns, such as ctid , have (arbitrary) negative numbers. |
|
attndims |
integer | 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 is an array.) |
|
attcacheoff |
integer | 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 |
integer | 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 it. |
|
attbyval |
boolean | 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 |
boolean | This represents a not-null constraint. | |
atthasdef |
boolean | 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 value. (Check attgenerated to determine whether this is a default or a generation expression.) |
|
atthasmissing |
boolean | 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 |
boolean | 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 |
boolean | This column is defined locally in the relation. Note that a column may be locally defined and inherited simultaneously. | |
attinhcount |
integer | 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 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. |
Parent topic: System Catalogs Definitions