The pg_largeobject system catalog table holds the data making up ‘large objects’. A large object is identified by an OID assigned when it is created. Each large object is broken into segments or 'pages’ small enough to be conveniently stored as rows in pg_largeobject. The amount of data per page is defined to be LOBLKSIZE (which is currently BLCKSZ/4, or typically 8K).

Each row of pg_largeobject holds data for one page of a large object, beginning at byte offset (pageno* LOBLKSIZE) within the object. The implementation allows sparse storage: pages may be missing, and may be shorter than LOBLKSIZE bytes even if they are not the last page of the object. Missing regions within a large object read as zeroes.

Table 1. pg_catalog.pg_largeobject

column type references description
loid oid   Identifier of the large object that includes this page.
pageno integer   Page number of this page within its large object (counting from zero).
data bytea   Actual data stored in the large object. This will never be more than LOBLKSIZE bytes and may be less.