Re: Storing Binary Data - Mailing list pgsql-odbc

Hello,

Unfortunately you will probably need to build the large object extension in.
In the source contrib directory there is a directory called lo.  Below is
the readme for this directory.  If you read it, it will explain the problem
(why oids don't work) and the solution. And of course how to install it.
Don't forget to run the lo.sql script on the database in question.

eg:

For my installation, I found the directory at:
/usr/src/postgresql-7.3.2/contrib/lo

Hope that helps ...

Mike.

README.lo

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

-----Original Message-----
      There is no data type lo like you mentioned in PGSQL. O.K.
Here I gave my table structure.

Table name photo:
ddocode  varchar  5
treasurycode  varchar  5
signature  bytea
photo1  bytea
photo2  bytea

     some times instead of bytea, I have used oid also.

>CREATE TABLE Student
>(
>StudentID integer NOT NULL,
>Picture lo,
>PRIMARY KEY( StudentID )
>);
>
>CREATE TRIGGER TRIGStudentPicture
>BEFORE UPDATE OR DElETE
>ON Student
>FOR EACH ROW
>EXECUTE PROCEDURE lo_manage(Picture);
>
>Make sure you are using the latest pgsql odbc driver from Hiroshi Inoue
>site as well.

--
Mike Miller,
Computer Programmer,
Department of Psychology,
University Of Otago
mike@psy.otago.ac.nz
+64 3 479 5402

...when you lay awake at night hoping that those elves from "The Elves and
the Shoemaker" know where you work and can program in C++ as well as they
can sew together sandles...
-- stolen from (http://www.gameai.com/youknow.html)