PostgreSQL type extension for managing Large Objects
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Overview
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 new PostgreSQL-specific applications, but existing ones
using JDBC or ODBC won't delete the objects, resulting in orphaning - objects
that are not referenced by anything, and simply occupy disk space.
The Fix
I've fixed this by creating a new data type 'lo', some support functions, and
a Trigger which handles the orphaning problem. 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 'lo' type was created because we needed to differentiate between plain
OIDs and Large Objects. Currently the JDBC driver handles this dilemma easily,
but (after talking to Byron), the ODBC driver needed a unique type. They had
created an 'lo' type, but not the solution to orphaning.
You don't actually 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.
Install
Ok, first build the shared library, and install. Typing 'make install' in the
contrib/lo directory should do it.
Then, as the postgres super user, run the lo.sql script in any database that
needs the features. This will install the type, and define the support
functions. You can run the script once in template1, and the objects will be
inherited by subsequently-created databases.
How to Use
The easiest way is by an example:
> create table image (title text, raster lo);
> create trigger t_raster before update or delete on image
> for each row execute procedure lo_manage(raster);
Create a trigger for each column that contains a lo type, and give the column
name as the trigger procedure argument. You can have more than one trigger on
a table if you need multiple lo columns in the same table, but don't forget to
give a different name to each trigger.
Issues
* Dropping a table will still orphan any objects it contains, as the trigger
is not executed.
Avoid this by preceding the 'drop table' with 'delete from {table}'.
If you already have, or suspect you have, orphaned large objects, see
the contrib/vacuumlo module to help you clean them up. It's a good idea
to run contrib/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.
As the ODBC driver needs a permanent lo type (& JDBC could be optimised to
use it if it's Oid is fixed), and as the above issues can only be fixed by
some internal changes, I feel it should become a permanent built-in type.
I'm releasing this into contrib, just to get it out, and tested.
Peter Mount <peter@retep.org.uk> June 13 1998
