And now everything runs smoothly: I had to use the bigger machine for creating and indexing the collection (now consisting of 677 documents with 1.9 GB input size resulting in a 2.1 GB collection).
Fine; does it mean that the index is now recognized by the optimizer? As Maximiliam stated (thanks!), the index won't be used if some match options are specified that don't match the index options. Since Version ~6.5, the default database index options will be assumed as default if the query doesn't contain any explicit options, so you'll just be fine by omitting the index options.
I would be interested -- mainly for development and debugging -- to access information concerning query processing (i.e., information displayed in the "Query Info" buffer in the gui) with the client. I use the Perl API.
That can be done by activating the QUERYINFO option and calling info(). A little example:
print $session->execute("set queryinfo on")."\n"; ... print $session->execute("xquery 1")."\n"; print $session->info()."\n"; ...
Hope this helps, Christian