> 4) The code which traverses predicate lists seems to always traverse
them
> in backwards order, e.g. this code from HashJoinStrategy.java:
>
> for (int i = storeRestrictionList.size() - 1; i >= 0; i--)
>
> Why do we always traverse these backwards? Is this just an
optimization
> in order to call the size() method only once? Or is there something
> deeper going on?
This is just habit more than anything. A lot of the optimizer-related
work that I've been doing requires the removal of certain predicates from
predicate lists at various points in the code, and in that case reverse
traversal is better (because removal of elements from the rear does not
require adjustments to the loop index). So I often write loop iteration
with backwards traversal out of sheer habit, even when removal of predicates
is not happening. If you think this makes the code more confusing or less
intuitive I have no problems with switching to forward traversal.
A small suggestion on this backwards traversal-- if you use the Iterator
class, you can achieve the same thing in a much more readable way. The
change can be as simple as making the Vector in QueryTreeNodeVector a List
and adding an iterator method in QTNV class.
