PostgreSQL 9.6.5 Documentation
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33.4. Server-side Functions

Server-side functions tailored for manipulating large objects from SQL are listed in Table 33-1.

Table 33-1. SQL-oriented Large Object Functions

FunctionReturn TypeDescriptionExampleResult
lo_from_bytea(loid oid, string bytea) oid Create a large object and store data there, returning its OID. Pass 0 to have the system choose an OID. lo_from_bytea(0, E'\\xffffff00')24528
lo_put(loid oid, offset bigint, str bytea) void Write data at the given offset. lo_put(24528, 1, E'\\xaa') 
lo_get(loid oid [, from bigint, for int]) bytea Extract contents or a substring thereof. lo_get(24528, 0, 3)\xffaaff

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, 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 superuser privilege.

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.

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