pg_shdepend records the dependency relationships between database objects and shared objects, such as roles. This information allows PostgreSQL to ensure that those objects are unreferenced before attempting to delete them.
pg_depend, which performs a similar function for dependencies involving objects within a single database.
Unlike most system catalogs,
pg_shdepend is shared across all databases of a cluster: there is only one copy of
pg_shdepend per cluster, not one per database.
The OID of the database the dependent object is in, or zero for a shared object
The OID of the system catalog the dependent object is in
The OID of the specific dependent object
For a table column, this is the column number (the
The OID of the system catalog the referenced object is in (must be a shared catalog)
The OID of the specific referenced object
A code defining the specific semantics of this dependency relationship; see text
In all cases, a
pg_shdepend entry indicates that the referenced object cannot be dropped without also dropping the dependent object. However, there are several subflavors identified by
The referenced object (which must be a role) is the owner of the dependent object.
The referenced object (which must be a role) is mentioned in the ACL (access control list, i.e., privileges list) of the dependent object. (A
SHARED_DEPENDENCY_ACL entry is not made for the owner of the object, since the owner will have a
SHARED_DEPENDENCY_OWNER entry anyway.)
The referenced object (which must be a role) is mentioned as the target of a dependent policy object.
The referenced object (which must be a tablespace) is mentioned as the tablespace for a relation that doesn't have storage.
Other dependency flavors might be needed in future. Note in particular that the current definition only supports roles and tablespaces as referenced objects.
As in the
pg_depend catalog, most objects created during initdb are considered “pinned”. No entries are made in
pg_shdepend that would have a pinned object as either referenced or dependent object.