Server-side functions tailored for manipulating large objects from SQL are listed in Table 35.1.
Table 35.1. SQL-Oriented Large Object Functions
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_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, 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_import
and 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_import
or 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_read
and
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 loread
and lowrite
.