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The pg_depend system catalog table 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 Greenplum system.
In all cases, a pg_depend entry indicates that the referenced object may not be dropped without also dropping the dependent object. However, there are several subflavors identified by deptype:
- DEPENDENCY_NORMAL (n) — A normal relationship between separately-created objects. The dependent object may be dropped without affecting the referenced object. The referenced object may 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.
- DEPENDENCY_AUTO (a) — 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.
- DEPENDENCY_INTERNAL (i) — 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.
DEPENDENCY_PIN (p) — 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 system initialization. The columns for the dependent
object contain zeroes.
Table 1. pg_catalog.pg_depend column type references description 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. 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 referenced column number. For all other object types, this column is zero. deptype char A code defining the specific semantics of this dependency relationship.