Using Large Objects

In Postgres, large objects (also known as blobs) are used to hold data in the database that cannot be stored in a normal SQL table. They are stored as a Table/Index pair, and are referred to from your own tables by an OID value.

For Postgres, you must access large objects within an SQL transaction. Although this has always been true in principle, it was not strictly enforced until the release of v6.5. You would open a transaction by using the setAutoCommit() method with an input parameter of false: Connection mycon; ... mycon.setAutoCommit(false); ... now use Large Objects

Now, there are two methods of using Large Objects. The first is the standard JDBC way, and is documented here. The other, uses our own extension to the api, which presents the libpq large object API to Java, providing even better access to large objects than the standard. Internally, the driver uses the extension to provide large object support.

In JDBC, the standard way to access them is using the getBinaryStream() method in ResultSet, and setBinaryStream() method in PreparedStatement. These methods make the large object appear as a Java stream, allowing you to use the java.io package, and others, to manipulate the object.

For example, suppose you have a table containing the file name of an image, and a large object containing that image:

create table images (imgname name,imgoid oid);
    

To insert an image, you would use:

File file = new File("myimage.gif");
FileInputStream fis = new FileInputStream(file);
PreparedStatement ps = conn.prepareStatement("insert into images values (?,?)");
ps.setString(1,file.getName());
ps.setBinaryStream(2,fis,file.length());
ps.executeUpdate();
ps.close();
fis.close();
    

Now in this example, setBinaryStream transfers a set number of bytes from a stream into a large object, and stores the OID into the field holding a reference to it.

Retrieving an image is even easier (I'm using PreparedStatement here, but Statement can equally be used):

PreparedStatement ps = con.prepareStatement("select oid from images where name=?");
ps.setString(1,"myimage.gif");
ResultSet rs = ps.executeQuery();
if(rs!=null) {
    while(rs.next()) {
        InputStream is = rs.getBinaryInputStream(1);
        // use the stream in some way here
        is.close();
    }
    rs.close();
}
ps.close();
    

Now here you can see where the Large Object is retrieved as an InputStream. You'll also notice that we close the stream before processing the next row in the result. This is part of the JDBC Specification, which states that any InputStream returned is closed when ResultSet.next() or ResultSet.close() is called.