| Postgres-XC 1.2 Documentation | ||||
|---|---|---|---|---|
| Prev | Fast Backward | Chapter 47. System Catalogs | Fast Forward | Next |
Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly. You can read PostgreSQL as Postgres-XC except for version number, which is specific to each product.
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 47-7. pg_attribute Columns
| Name | Type | References | Description |
|---|---|---|---|
| attrelid | oid | pg_class.oid | The table this column belongs to |
| attname | name | The column name | |
| atttypid | oid | pg_type.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 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. 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 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 | pg_collation.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 |
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.