51.7. 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 51.7.
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.