PostgreSQL 9.3.2 Documentation
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# 47.13. `pg_constraint`

The catalog `pg_constraint` stores check, primary key, unique, foreign key, and exclusion 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, not here.

User-defined constraint triggers (created with `CREATE CONSTRAINT TRIGGER`) also give rise to an entry in this table.

Check constraints on domains are stored here, too.

Table 47-13. `pg_constraint` Columns

Name | Type | References | Description
--- | --- | --- | ---
`oid` | `oid` |  | Row identifier (hidden attribute; must be explicitly selected)
`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, `t` = constraint trigger, `x` = exclusion constraint
`condeferrable` | `bool` |  | Is the constraint deferrable?
`condeferred` | `bool` |  | Is the constraint deferred by default?
`convalidated` | `bool` |  | Has the constraint been validated? Currently, can only be false for foreign keys and CHECK constraints
`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
`conindid` | `oid` | ``pg_class`.oid` | The index supporting this constraint, if it's a unique, primary key, foreign key, or exclusion constraint; else 0
`confrelid` | `oid` | ``pg_class`.oid` | If a foreign key, the referenced table; else 0
`confupdtype` | `char` |  | Foreign key update action code: `a` = no action, `r` = restrict, `c` = cascade, `n` = set null, `d` = set default
`confdeltype` | `char` |  | Foreign key deletion action code: `a` = no action, `r` = restrict, `c` = cascade, `n` = set null, `d` = set default
`confmatchtype` | `char` |  | Foreign key match type: `f` = full, `p` = partial, `s` = simple
`conislocal` | `bool` |  | This constraint is defined locally for the relation. Note that a constraint can be locally defined and inherited simultaneously.
`coninhcount` | `int4` |  | The number of direct inheritance ancestors this constraint has. A constraint with a nonzero number of ancestors cannot be dropped nor renamed.
`connoinherit` | `bool` |  | This constraint is defined locally for the relation. It is a non-inheritable constraint.
`conkey` | `int2[]` | ``pg_attribute`.attnum` | If a table constraint (including foreign keys, but not constraint triggers), list of the constrained columns
`confkey` | `int2[]` | ``pg_attribute`.attnum` | If a foreign key, list of the referenced columns
`conpfeqop` | `oid[]` | ``pg_operator`.oid` | If a foreign key, list of the equality operators for PK = FK comparisons
`conppeqop` | `oid[]` | ``pg_operator`.oid` | If a foreign key, list of the equality operators for PK = PK comparisons
`conffeqop` | `oid[]` | ``pg_operator`.oid` | If a foreign key, list of the equality operators for FK = FK comparisons
`conexclop` | `oid[]` | ``pg_operator`.oid` | If an exclusion constraint, list of the per-column exclusion operators
`conbin` | `pg_node_tree` |  | If a check constraint, an internal representation of the expression
`consrc` | `text` |  | If a check constraint, a human-readable representation of the expression

In the case of an exclusion constraint, `conkey` is only useful for constraint elements that are simple column references. For other cases, a zero appears in `conkey` and the associated index must be consulted to discover the expression that is constrained. (`conkey` thus has the same contents as `pg_index`.`indkey` for the index.)

> 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 each relation.

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