mirror of https://github.com/postgres/postgres
70 lines
2.6 KiB
Plaintext
70 lines
2.6 KiB
Plaintext
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PostgreSQL type extension for managing Large Objects
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----------------------------------------------------
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Overview
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One of the problems with the JDBC driver (and this affects the ODBC driver
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also), is that the specification assumes that references to BLOBS (Binary
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Large OBjectS) are stored within a table, and if that entry is changed, the
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associated BLOB is deleted from the database.
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As PostgreSQL stands, this doesn't occur. It allocates an OID for each object,
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and it is up to the application to store, and ultimately delete the objects.
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Now this is fine for new postgresql specific applications, but existing ones
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using JDBC or ODBC wont delete the objects, arising to orphaning - objects
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that are not referenced by anything, and simply occupy disk space.
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The Fix
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I've fixed this by creating a new data type 'lo', some support functions, and
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a Trigger which handles the orphaning problem.
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The 'lo' type was created because we needed to differenciate between normal
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Oid's and Large Objects. Currently the JDBC driver handles this dilema easily,
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but (after talking to Byron), the ODBC driver needed a unique type. They had created an 'lo' type, but not the solution to orphaning.
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Install
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Ok, first build the shared library, and install. Typing 'make install' in the
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contrib/lo directory should do it.
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Then, as the postgres super user, run the lo.sql script. This will install the
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type, and define the support functions.
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How to Use
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The easiest way is by an example:
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> create table image (title text,raster lo);
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> create trigger t_image before update or delete on image for each row execute procedure lo_manage(raster);
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Here, a trigger is created for each column that contains a lo type.
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Issues
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* dropping a table will still orphan any objects it contains, as the trigger
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is not actioned.
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For now, precede the 'drop table' with 'delete from {table}'. However, this
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could be fixed by having 'drop table' perform an additional
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'select lo_unlink({colname}::oid) from {tablename}'
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for each column, before actually dropping the table.
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* Some frontends may create their own tables, and will not create the
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associated trigger(s). Also, users may not remember (or know) to create
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the triggers.
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This can be solved, but would involve changes to the parser.
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As the ODBC driver needs a permanent lo type (& JDBC could be optimised to
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use it if it's Oid is fixed), and as the above issues can only be fixed by
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some internal changes, I feel it should become a permanent built-in type.
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I'm releasing this into contrib, just to get it out, and tested.
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Peter Mount <peter@retep.org.uk> June 13 1998
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