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ObjectAddress

Location

src/include/catalog/objectaddress.h:24-29

Overview

ObjectAddress is a fundamental data structure that represents any database object in PostgreSQL, providing a unified way to identify tables, functions, types, constraints, and other database entities through a three-component addressing scheme.

Definition

typedef struct ObjectAddress
{
    Oid         classId;        /* Class Id from pg_class */
    Oid         objectId;       /* OID of the object */
    int32       objectSubId;    /* Subitem within object (eg column), or 0 */
} ObjectAddress;

Detailed Description

ObjectAddress serves as PostgreSQL's universal object identifier, enabling the system to reference any database object in a consistent manner. This structure is essential for dependency tracking, permission checking, object manipulation, and catalog operations. The three-field design allows for hierarchical addressing where objects can have sub-objects (like columns within tables).

The structure is extensively used throughout PostgreSQL's internal systems, particularly in:

  • Dependency management and tracking
  • Object ownership and permission verification
  • DDL operations and object manipulation
  • Catalog system operations
  • Object description and identity resolution

Parameters / Member Variables

  • classId: The OID of the system catalog that contains this object (corresponds to pg_class entries for different object types)
  • objectId: The unique OID of the specific object within its catalog
  • objectSubId: Sub-object identifier within the main object (e.g., column number for table attributes), or 0 for the object itself

Dependencies

Notes and Other Information

  • An InvalidObjectAddress constant is provided for initialization and error conditions
  • Helper macros ObjectAddressSet and ObjectAddressSubSet are available for convenient initialization
  • The classId typically corresponds to system catalog relation OIDs (e.g., RelationRelationId, ProcedureRelationId)
  • A zero objectSubId indicates the entire object; non-zero values indicate specific sub-components
  • This structure is central to PostgreSQL's object dependency system and is used throughout the codebase for object management operations
  • The design allows PostgreSQL to maintain referential integrity and implement features like CASCADE operations across different object types