I have a table called tblA and its having two columns
id_ (numeric 50,0), obj_ oid
For example, tblA having 1 entries
id_ | obj_
1 | 1001
In pg_largeobject table, there are three entries against loid 1001 (3 page nos) which is fine. But, pg_largeobject table is having three columns (
loid | pageno | data
(oid) (integer) (bytea)
I read about oids and bytea, both are different data types and being used in that way. But I am confused about pg_largeobject table structure, why its using both data types for same data?
Is postgres internally storing oids as bytea, and just returning oids for reference? If its working in this manner, why can't use just numeric type for reference instead of oid type.
If not, what is the use of oid here and is any data sitting behind the oid somewhere in the database, and what is the bytea column data about.?
Best Answer
oid
as a type is a 32-bit unsigned integer that stands forobject id
, and is used by the system as an all-purpose surrogate key to refer to different objects and sometimes rows inpg_catalog
. Being the type for keys to large objects is just one of its many uses. Other "objects" like tables, sequences, types and more are refered to by theiroid
.It stores the contents of large objects as small bytea pages (~2000 bytes)in the
pg_largeobject
table, withloid
as the ID or the large object, and(loid,pageno)
as the unique key for that table. The generator of new values forloid
can be compared to a integer sequence, except that after wrapping around at2^32
, it can avoid conflicts with existing values. As users, we don't have to care about this, it's managed by the internals.The oid is used as a handle and the bytea column holds the actual binary data.