This chapter describes the implementation and the programming and query language interfaces to PostgreSQL large object data. We use the libpq C library for the examples in this chapter, but most programming interfaces native to PostgreSQL support equivalent functionality. Other interfaces may use the large object interface internally to provide generic support for large values. This is not described here.
POSTGRES 4.2, the indirect predecessor of PostgreSQL, supported three standard implementations of large objects: as files external to the POSTGRES server, as external files managed by the POSTGRES server, and as data stored within the POSTGRES database. This caused considerable confusion among users. As a result, only support for large objects as data stored within the database is retained in PostgreSQL. Even though this is slower to access, it provides stricter data integrity. For historical reasons, this storage scheme is referred to as Inversion large objects. (You will see the term Inversion used occasionally to mean the same thing as large object.) Since PostgreSQL 7.1, all large objects are placed in one system table called pg_largeobject.
PostgreSQL 7.1 introduced a mechanism (nicknamed "TOAST") that allows data values to be much larger than single pages. This makes the large object facility partially obsolete. One remaining advantage of the large object facility is that it allows values up to 2 GB in size, whereas TOASTed fields can be at most 1 GB. Also, large objects can be manipulated piece-by-piece much more easily than ordinary data fields, so the practical limits are considerably different.
