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.