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title: pg_constraint
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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.html#topic1) catalog table. Check constraints on domains are stored here, too.
<a id="topic1__gh143898"></a>
<span class="tablecap">Table 1. pg\_catalog.pg\_constraint</span>
| 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.oid | The table this constraint is on; 0 if not a table constraint. |
| `contypid ` | oid | pg\_type.oid | The domain this constraint is on; 0 if not a domain constraint. |
| `confrelid` | oid | pg\_class.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. |