The catalog pg_largeobject 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 2 kB).
Table 44-23. pg_largeobject Columns
|loid||oid||Identifier of the large object that includes this page|
|pageno||int4||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 might be less|
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 might be missing, and might 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.