53.7. 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 53.7.
pg_attribute
Columns
Column Type Description |
---|
The table this column belongs to |
The column name |
The data type of this column (zero for a dropped column) |
A copy of
|
The number of the column. Ordinary columns are numbered from 1
up. System columns, such as
|
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 |
|
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 " .) |
A copy of
|
A copy of
|
Normally a copy of
|
The current compression method of the column. Typically this is
|
This represents a not-null constraint. |
This column has a default expression or generation expression, in which
case there will be a corresponding entry in the
|
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
|
If a zero byte (
|
If a zero byte (
|
This column has been dropped and is no longer valid.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. |
This column is defined locally in the relation. Note that a column can be locally defined and inherited simultaneously. |
The number of direct ancestors this column has. A column with a nonzero number of ancestors cannot be dropped nor renamed. |
|
The defined collation of the column, or zero if the column is not of a collatable data type |
Column-level access privileges, if any have been granted specifically on this column |
Attribute-level options, as " keyword=value " strings |
Attribute-level foreign data wrapper options, as " keyword=value " strings |
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
|
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.