The catalog pg_depend records the dependency relationships between database objects. This information allows DROP commands to find which other objects must be dropped by DROP CASCADE or prevent dropping in the DROP RESTRICT case.
See also pg_shdepend, which performs a similar function for dependencies involving objects that are shared across a database cluster.
Table 45-17. pg_depend Columns
|classid||oid||pg_class.oid||The OID of the system catalog the dependent object is in|
|objid||oid||any OID column||The OID of the specific dependent object|
|objsubid||int4||For a table column, this is the column number (the objid and classid refer to the table itself). For all other object types, this column is zero.|
|refclassid||oid||pg_class.oid||The OID of the system catalog the referenced object is in|
|refobjid||oid||any OID column||The OID of the specific referenced object|
|refobjsubid||int4||For a table column, this is the column number (the refobjid and refclassid refer to the table itself). For all other object types, this column is zero.|
|deptype||char||A code defining the specific semantics of this dependency relationship; see text|
In all cases, a pg_depend entry indicates that the referenced object cannot be dropped without also dropping the dependent object. However, there are several subflavors identified by deptype:
A normal relationship between separately-created objects. The dependent object can be dropped without affecting the referenced object. The referenced object can only be dropped by specifying CASCADE, in which case the dependent object is dropped, too. Example: a table column has a normal dependency on its data type.
The dependent object can be dropped separately from the referenced object, and should be automatically dropped (regardless of RESTRICT or CASCADE mode) if the referenced object is dropped. Example: a named constraint on a table is made autodependent on the table, so that it will go away if the table is dropped.
The dependent object was created as part of creation of the referenced object, and is really just a part of its internal implementation. A DROP of the dependent object will be disallowed outright (we'll tell the user to issue a DROP against the referenced object, instead). A DROP of the referenced object will be propagated through to drop the dependent object whether CASCADE is specified or not. Example: a trigger that's created to enforce a foreign-key constraint is made internally dependent on the constraint's pg_constraint entry.
There is no dependent object; this type of entry is a signal that the system itself depends on the referenced object, and so that object must never be deleted. Entries of this type are created only by initdb. The columns for the dependent object contain zeroes.
Other dependency flavors might be needed in future.