postgresql/contrib/lo
Tom Lane d08741eab5 Restructure the key include files per recent pghackers discussion: there
are now separate files "postgres.h" and "postgres_fe.h", which are meant
to be the primary include files for backend .c files and frontend .c files
respectively.  By default, only include files meant for frontend use are
installed into the installation include directory.  There is a new make
target 'make install-all-headers' that adds the whole content of the
src/include tree to the installed fileset, for use by people who want to
develop server-side code without keeping the complete source tree on hand.
Cleaned up a whole lot of crufty and inconsistent header inclusions.
2001-02-10 02:31:31 +00:00
..
lo_drop.sql Add missing /contrib files to CVS. 2000-06-15 19:05:22 +00:00
lo_test.sql Add missing /contrib files to CVS. 2000-06-15 19:05:22 +00:00
lo.c Restructure the key include files per recent pghackers discussion: there 2001-02-10 02:31:31 +00:00
lo.sql.in Make the world safe for unsigned OIDs. 2000-11-21 21:51:58 +00:00
Makefile Add support for VPATH builds, that is, building somewhere else than in the 2000-10-20 21:04:27 +00:00
README.lo Add missing /contrib files 2000-06-19 14:02:16 +00:00

PostgreSQL type extension for managing Large Objects
----------------------------------------------------

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. It allocates an OID for each object,
and it is up to the application to store, and ultimately delete the objects.

Now this is fine for new postgresql specific applications, but existing ones
using JDBC or ODBC wont delete the objects, arising to 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 'lo' type was created because we needed to differenciate between normal
Oid's and Large Objects. Currently the JDBC driver handles this dilema 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.

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. This will install the
type, and define the support functions.

How to Use

The easiest way is by an example:

> create table image (title text,raster lo);
> create trigger t_image before update or delete on image for each row execute procedure lo_manage(raster);

Here, a trigger is created for each column that contains a lo type.

Issues

* dropping a table will still orphan any objects it contains, as the trigger
  is not actioned.

  For now, precede the 'drop table' with 'delete from {table}'. However, this
  could be fixed by having 'drop table' perform an additional

      'select lo_unlink({colname}::oid) from {tablename}'

  for each column, before actually dropping the table.

* 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.

  This can be solved, but would involve changes to the parser.

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