There are server-side functions callable from SQL that correspond to
each of the client-side functions described above; indeed, for the
most part the client-side functions are simply interfaces to the
equivalent server-side functions. The ones that are actually useful
to call via SQL commands are
lo_creat,
lo_create,
lo_unlink,
lo_import, and
lo_export.
Here are examples of their use:
CREATE TABLE image (
name text,
raster oid
);
SELECT lo_creat(-1); -- returns OID of new, empty large object
SELECT lo_create(43213); -- attempts to create large object with OID 43213
SELECT lo_unlink(173454); -- deletes large object with OID 173454
INSERT INTO image (name, raster)
VALUES ('beautiful image', lo_import('/etc/motd'));
INSERT INTO image (name, raster) -- same as above, but specify OID to use
VALUES ('beautiful image', lo_import('/etc/motd', 68583));
SELECT lo_export(image.raster, '/tmp/motd') FROM image
WHERE name = 'beautiful image';
The server-side lo_import and
lo_export functions behave considerably differently
from their client-side analogs. These two functions read and write files
in the server's file system, using the permissions of the database's
owning user. Therefore, their use is restricted to superusers. In
contrast, the client-side import and export functions read and write files
in the client's file system, using the permissions of the client program.
The client-side functions can be used by any
PostgreSQL user.
