mirror of https://github.com/postgres/postgres
72 lines
2.6 KiB
Plaintext
Executable File
72 lines
2.6 KiB
Plaintext
Executable File
PostgreSQL type extension for managing Large Objects
|
|
----------------------------------------------------
|
|
|
|
$Id: README,v 1.1 1998/06/16 07:07:11 momjian Exp $
|
|
|
|
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
|
|
|