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.
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 LightDB 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 LightDB-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 domain of 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.
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.
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
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.
Peter Mount <peter@retep.org.uk>