| F.27. lo | ||||
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F.27. lo
Thelo module provides support for managing Large Objects (also called LOs or BLOBs). This includes a data typelo and a triggerlo_manage.
This module is considered“trusted”, that is, it can be installed by non-superusers who haveCREATE privilege on the current database.
F.27.1. Rationale
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.
AsPostgres Pro 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 forPostgres Pro-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.
Thelo module allows fixing this by attaching a trigger to tables that contain LO reference columns. The trigger essentially just does alo_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 typelo, which is really just a domain of theoid 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 thelo 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 uselo for BLOB columns.
F.27.2. How to Use It
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 aBEFORE 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 usingBEFORE UPDATE OFcolumn_name. If you need multiplelo 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.27.3. Limitations
Dropping a table will still orphan any objects it contains, as the trigger is not executed. You can avoid this by preceding the
DROP TABLEwithDELETE FROM.tableTRUNCATEhas the same hazard.If you already have, or suspect you have, orphaned large objects, see thevacuumlo module to help you clean them up. It's a good idea to runvacuumlo occasionally as a back-stop to the
lo_managetrigger.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.27.4. Author
Peter Mount<peter@retep.org.uk>