49.7. 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 49.7. pg_attribute
Columns
Column Type Description |
---|
The table this column belongs to |
The column name |
The data type of this column |
|
A copy of |
The number of the column. Ordinary columns are numbered from 1 up. System columns, such as |
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”.) |
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 |
|
A copy of |
Normally a copy of |
A copy of |
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This column has been dropped and is no longer valid.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.