1.1. Logical Structure of Database Cluster

A database cluster is a collection of databases managed by a PostgreSQL server. If you are hearing this definition for the first time, you might be wondering what it means. The term ‘database cluster’ in PostgreSQL does not mean ‘a group of database servers’. A PostgreSQL server runs on a single host and manages a single database cluster.

Figure 1.1 shows the logical structure of a database cluster. A database is a collection of database objects. In the relational database theory, a database object is a data structure used to store or reference data. A (heap) table is a typical example, and there are many others, such as indexes, sequences, views, functions. In PostgreSQL, databases themselves are also database objects and are logically separated from each other. All other database objects (e.g., tables, indexes, etc) belong to their respective databases.

All the database objects in PostgreSQL are internally managed by respective object identifiers (OIDs), which are unsigned 4-byte integers. The relations between database objects and their respective OIDs are stored in appropriate system catalogs, depending on the type of objects. For example, OIDs of databases and heap tables are stored in pg_database and pg_class, respectively.

To find the desired OIDs, execute the following queries:

sampledb=# SELECT datname, oid FROM pg_database WHERE datname = 'sampledb';
 datname  |  oid
----------+-------
 sampledb | 16384
(1 row)

sampledb=# SELECT relname, oid FROM pg_class WHERE relname = 'sampletbl';
  relname  |  oid
-----------+-------
 sampletbl | 18740
(1 row)

testdb=# SELECT version();
                                                   version
-------------------------------------------------------------------------------------------------------------
 PostgreSQL 8.0.26 on arm-apple-darwin24.5.0, compiled by GCC Apple clang version 17.0.0 (clang-1700.0.13.5)
(1 row)

testdb=# \d tbl
      Table "public.tbl"
 Column |  Type   | Modifiers
--------+---------+-----------
 id     | integer |
 data   | text    |

testdb=# SELECT * FROM tbl;
 id | data
----+------
  1 | a
  2 | b
  3 | c
(3 rows)

testdb=# SELECT oid, * FROM tbl;
  oid  | id | data
-------+----+------
 17236 |  1 | a
 17237 |  2 | b
 17238 |  3 | c
(3 rows)