PostgreSQL 8.1.4 Documentation
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# 42.13. `pg_constraint`

The catalog `pg_constraint` stores check, primary key, unique, 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` catalog.

Check constraints on domains are stored here, too.

Table 42-13. `pg_constraint` Columns

Name | Type | References | Description
--- | --- | --- | ---
`conname` | `name` |  | Constraint name (not necessarily unique!)
`connamespace` | `oid` | ``pg_namespace`.oid` | The OID of the namespace that contains this constraint
`contype` | `char` |  | `c` = check constraint, `f` = foreign key constraint, `p` = primary key constraint, `u` = unique constraint
`condeferrable` | `bool` |  | Is the constraint deferrable?
`condeferred` | `bool` |  | 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` | `int2[]` | ``pg_attribute`.attnum` | If a table constraint, list of columns which the constraint constrains
`confkey` | `int2[]` | ``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

> Note: `consrc` is not updated when referenced objects change; for example, it won't track renaming of columns. Rather than relying on this field, it's best to use `pg_get_constraintdef()` to extract the definition of a check constraint.

> Note: `pg_class.relchecks` needs to agree with the number of check-constraint entries found in this table for the given relation.

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