lter clauses) are valid. As soon as one of the
clauses finishes stepping through its database (sub)tree, the whole call to
<code>select</code> will terminate, because further values returned from other
generator clauses cannot be part of the result set.

<p>Therefore, <code>select</code> would find all results most quickly if it
could simply call only the generator clause with the smallest (sub)tree.
Unfortunately, this is usually not known in advance. It depends on the
distribution of the data in the datab