| PostgreSQL 7.4.8 Documentation | ||||
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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 other objects. See pg_class.)
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 | 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 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. | |
| attisset | bool | If true, this attribute is a set. In that case, what is really stored in the attribute is the OID of a row in the pg_proc catalog. The pg_proc row contains the query string that defines this set, i.e., the query to run to get the set. So the atttypid (see above) refers to the type returned by this query, but the actual length of this attribute is the length (size) of an oid. --- At least this is the theory. All this is probably quite broken these days. | |
| 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. |