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The pg_shdepend system catalog table records the dependency relationships between database objects and shared objects, such as roles. This information allows Greenplum Database to ensure that those objects are unreferenced before attempting to delete them. See also 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 Greenplum system: there is only one copy of pg_shdepend per system, not one per database.
In all cases, a pg_shdepend entry indicates that the referenced object may not be dropped without also dropping the dependent object. However, there are several subflavors identified by deptype:
- SHARED_DEPENDENCY_OWNER (o) — The referenced object (which must be a role) is the owner of the dependent object.
- SHARED_DEPENDENCY_ACL (a) — The referenced object (which must be a role) is mentioned in the ACL (access control list) of the dependent object.
SHARED_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_shdepend column type references description dbid oid pg_database.oid The OID of the database the dependent object is in, or zero for a shared object. 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 (must be a shared catalog). 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.