Xquery Problem for for takes really long time

Question

I am having problems with an issue using doing an xml xQuery from Oxygen to a dbxml database.
I can't list the code because it is quite detailed and on a secure computer, but the concept is simple.
I have an xquery that looks like this:
Bunch of let statements
<results>
Let $y := "one"
For $thing in $list_of_things
More let statements
Where ....
Return
Html and more embedded xquery expressions including for, where etc.
</results>
I run a query with $y := "one" as above and it takes 1.5 seconds.
If I modify the query slightly by replacing the "let $y := "one" with an additional "for" it takes 250 seconds, though the results are the same as before.
Why did it take so long?
All I did was add the for which only has one item in it so it should do the same thing?!
Modified Query
Bunch of let statements
<results>
For $y in ("one")
For $thing in $list_of_things
More let statements
Where ....
Return
Html and more embedded xquery expressions including for, where etc.
</results>
Thanks for any help anyone can offer, Curt

Answer 1

Curt, perhaps you'd need to provide a bit more relevant detail and in simpler form in order to get a helpful answer. On the face of it, changing let to for in that position might have an impact simply because you are changing a simple variable assignment followed by a for clause to a nested for...for clause. Is your "one" value a dummy for demonstration purposes or the actual value in your code? On the face of it, it looks like for each loop of the first for, you'd be repeating the execution of all the subsequent code. Without more context it's difficult to judge what would happen exactly.
I suspect that if it's XQuery you're needing help with you'll get more responses on a dedicated forum. For help with DBXML performance issues you're probably going to need to tell people about your indices and be more explicit about their relationship to your query, in order for them to understand the problem.
If that's not helpful just review your code again and continue experimenting.
Tim