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  5. PG_SHDEPEND

PG_SHDEPEND

PG_SHDEPEND records the dependency relationships between database objects and shared objects, such as roles. Based on this information, DWS can 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 a cluster: there is only one copy of PG_SHDEPEND per cluster, not one per database.

Table 1 PG_SHDEPEND columns

Name

Type

Reference

Description

dbid

oid

PG_DATABASE.oid

OID of the database the dependent object is in, or zero for a shared object

classid

oid

PG_CLASS.oid

OID of the system catalog the dependent object is in.

objid

oid

Any OID column

OID of the specific dependent object

objsubid

integer

-

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

OID of the system catalog the referenced object is in (must be a shared catalog)

refobjid

oid

Any OID column

OID of the specific referenced object

deptype

"char"

-

Code segment defining the specific semantics of this dependency relationship. See the following text for details.

objfile

text

-

Path of the user-defined C function library file.

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 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, that is, 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.

  • 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 initdb. The columns for the dependent object contain zeroes.