In Postgres, data values are stored in tuples and individual tuples cannot span data pages. Since the size of a data page is 8192 bytes, the upper limit on the size of a data value is relatively low. To support the storage of larger atomic values, Postgres provides a large object interface. This interface provides file oriented access to user data that has been declared to be a large type. This section describes the implementation and the programming and query language interfaces to Postgres large object data.
Originally, Postgres 4.2 supported three standard implementations of large objects: as files external to Postgres, as external files managed by Postgres, and as data stored within the Postgres database. It causes considerable confusion among users. As a result, we only support large objects as data stored within the Postgres database in PostgreSQL. Even though it is slower to access, it provides stricter data integrity. For historical reasons, this storage scheme is referred to as Inversion large objects. (We will use Inversion and large objects interchangeably to mean the same thing in this section.) Since PostgreSQL 7.1 all large objects are placed in one system table called pg_largeobject.