# pg_constraint

The `pg_constraint` system catalog table stores check and foreign key constraints on tables. Column constraints are not treated specially. Every column constraint is equivalent to some table constraint. Not-null constraints are represented in the [pg_attribute](./pg_attribute.md) catalog table. Check constraints on domains are stored here, too.

Table 1. `pg_catalog.pg_constraint`

| column           | type       | references            | description                                                  |
| :--------------- | :--------- | :-------------------- | :----------------------------------------------------------- |
| `conname`        | name       |                       | Constraint name                                              |
| `connamespace`   | oid        | pg_namespace.oid      | The OID of the namespace (schema) that contains this constraint. |
| `contype`        | char       |                       | `c` = check constraint, `f` = foreign key constraint.        |
| `condeferrable`  | boolean    |                       | Is the constraint deferrable?                                |
| `condeferred`    | boolean    |                       | Is the constraint deferred by default?                       |
| `conrelid`       | oid        | pg_class_internal.oid | The table this constraint is on; 0 if not a table constraint. |
| `conrootrelid  ` | oid        |                       | Root OID of the table this constraint is on;                 |
| `contypid`       | oid        | pg_type.oid           | The domain this constraint is on; 0 if not a domain constraint. |
| `confrelid`      | oid        | pg_class_internal.oid | If a foreign key, the referenced table; else 0.              |
| `confupdtype`    | char       |                       | Foreign key update action code.                              |
| `confdeltype`    | char       |                       | Foreign key deletion action code.                            |
| `confmatchtype`  | char       |                       | Foreign key match type.                                      |
| `conkey`         | smallint[] | pg_attribute.attnum   | If a table constraint, list of columns which the constraint constrains. |
| `confkey`        | smallint[] | pg_attribute.attnum   | If a foreign key, list of the referenced columns.            |
| `conbin`         | text       |                       | If a check constraint, an internal representation of the expression. |
| `consrc`         | text       |                       | If a check constraint, a human-readable representation of the expression. This is not updated when referenced objects change; for example, it won’t track renaming of columns. Rather than relying on this field, it is best to use `pg_get_constraintdef()` to extract the definition of a check constraint. |