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 |
|
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 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.