Server-side functions tailored for manipulating large objects from SQL are listed in Table 34.1.
Table 34.1. SQL-oriented Large Object Functions
||Create a large object and store data there, returning its OID. Pass
||Write data at the given offset.||
||Extract contents or a substring thereof.||
There are additional server-side functions corresponding to each of the client-side functions described earlier; indeed, for the most part the client-side functions are simply interfaces to the equivalent server-side functions. The ones just as convenient to call via SQL commands are
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';
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, by default 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 do not require any database privileges, except the privilege to read or write the large object in question.
It is possible to GRANT use of the server-side
lo_export functions to non-superusers, but careful consideration of the security implications is required. A malicious user of such privileges could easily parlay them into becoming superuser (for example by rewriting server configuration files), or could attack the rest of the server's file system without bothering to obtain database superuser privileges as such. Access to roles having such privilege must therefore be guarded just as carefully as access to superuser roles. Nonetheless, if use of server-side
lo_export is needed for some routine task, it's safer to use a role with such privileges than one with full superuser privileges, as that helps to reduce the risk of damage from accidental errors.
The functionality of
lo_write is also available via server-side calls, but the names of the server-side functions differ from the client side interfaces in that they do not contain underscores. You must call these functions as