For formatted documentation, please see https://www.postgresql.org/docs/16/lo.html.
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F.22. lo -- manage large objects
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F.22. lo -- manage large objects [7]#
[8]F.22.1. Rationale
[9]F.22.2. How to Use It
[10]F.22.3. Limitations
[11]F.22.4. Author
The lo module provides support for managing Large Objects (also called
LOs or BLOBs). This includes a data type lo and a trigger lo_manage.
This module is considered "trusted", that is, it can be installed by
non-superusers who have CREATE privilege on the current database.
F.22.1. Rationale [12]#
One of the problems with the JDBC driver (and this affects the ODBC
driver also), is that the specification assumes that references to
BLOBs (Binary Large OBjects) are stored within a table, and if that
entry is changed, the associated BLOB is deleted from the database.
As PostgreSQL stands, this doesn't occur. Large objects are treated as
objects in their own right; a table entry can reference a large object
by OID, but there can be multiple table entries referencing the same
large object OID, so the system doesn't delete the large object just
because you change or remove one such entry.
Now this is fine for PostgreSQL-specific applications, but standard
code using JDBC or ODBC won't delete the objects, resulting in orphan
objects -- objects that are not referenced by anything, and simply
occupy disk space.
The lo module allows fixing this by attaching a trigger to tables that
contain LO reference columns. The trigger essentially just does a
lo_unlink whenever you delete or modify a value referencing a large
object. When you use this trigger, you are assuming that there is only
one database reference to any large object that is referenced in a
trigger-controlled column!
The module also provides a data type lo, which is really just a
[13]domain over the oid type. This is useful for differentiating
database columns that hold large object references from those that are
OIDs of other things. You don't have to use the lo type to use the
trigger, but it may be convenient to use it to keep track of which
columns in your database represent large objects that you are managing
with the trigger. It is also rumored that the ODBC driver gets confused
if you don't use lo for BLOB columns.
F.22.2. How to Use It [14]#
Here's a simple example of usage:
CREATE TABLE image (title text, raster lo);
CREATE TRIGGER t_raster BEFORE UPDATE OR DELETE ON image
FOR EACH ROW EXECUTE FUNCTION lo_manage(raster);
For each column that will contain unique references to large objects,
create a BEFORE UPDATE OR DELETE trigger, and give the column name as
the sole trigger argument. You can also restrict the trigger to only
execute on updates to the column by using BEFORE UPDATE OF column_name.
If you need multiple lo columns in the same table, create a separate
trigger for each one, remembering to give a different name to each
trigger on the same table.
F.22.3. Limitations [15]#
* Dropping a table will still orphan any objects it contains, as the
trigger is not executed. You can avoid this by preceding the DROP
TABLE with DELETE FROM table.
TRUNCATE has the same hazard.
If you already have, or suspect you have, orphaned large objects,
see the [16]vacuumlo module to help you clean them up. It's a good
idea to run vacuumlo occasionally as a back-stop to the lo_manage
trigger.
* Some frontends may create their own tables, and will not create the
associated trigger(s). Also, users may not remember (or know) to
create the triggers.
F.22.4. Author [17]#
Peter Mount <[18][email protected]>
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