Thanks for the answer. I carefully read that document :-)

If I correctly understood, it may be better to disable autocommit.
So, when I'm querying a huge datbase with a SELECT statement that
returns hundred of thousand rows, when to do the commit? Between
each row retrivial and after a heuristic and prefined number (10,
100, 1000...)? What is the best practice?
Our database is composed of rows storing an XML file within a BLOB,
which may be huge.