how does postgresql handle LOB/CLOB/BLOB column data that dies before the query ends

Hi

Hacker from another open-source DB here (h2database.com).

How does postgresql handle the following situation?

(1) a table containing a LOB column
(2) a query that does
ResultSet rs = query("select lob_column from table_foo");
while (rs.next())
{
retrieve_lob_data(rs.getLob(1));
.... very long running stuff here......
}

In the face of concurrent updates that might overwrite the existing LOB
data, how does PostgresQL handle this?

Does it keep the LOB data around until the ResultSet/Connection is closed?
Or does it impose some extra constraint on the client side? e.g..
explicitly opening and closing a transaction, and only wipe the "old" LOB
data when the transaction is closed?

I ask because I have implemented two of the four LOB implementations that
H2 has used, and we are still having trouble :-(

Regards, Noel.