[basex-talk] Same query, huge difference in performance

[Paul Swennenhuis]

Hi,

I have two XQueries that do exactly the same thing, i.e. create a list 
of waters in the Facts database and display where these waters flow 
into, if any.
But Query  1 executes 10 times faster than Query 2 (in the GUI).
The only difference is in the selection of the query context.
Query 1 assumes the Facts database is open and does not use an explicit 
doc() or collection() context.
Query 2 uses an explicit collection("Facts") context
Query 1 executes in appr. 600 msecs, Query 2 in appr. 60 msecs (!)

How can there be such a huge difference? Is it because Query 2 operates 
twice on the $facts variable and does not need to evaluate that again?

Listings:

Query1

(: list waters and where they stream to (if any):)
for $source in  //(sea|river|lake)
let $toId := $source/to/@water
let $to := (//sea|//river|//lake)[@id=$toId][1]
let $name := if (empty($to/local-name())) then "none" else $to/local-name()
return
element water {
  element {$source/local-name()} {data($source/@name)},
  if (not($name="none"))then
  element streamsTo {
    attribute {$name} {data($to/@name)}
  }
  else ()
}

Query2

(: list waters and where they stream to (if any):)
let $facts :=  collection("Facts")//(sea|river|lake )
for $source in $facts
let $toId := $source/to/@water
let $to := $facts[@id=$toId][1]
let $name := if (empty($to/local-name())) then "none" else $to/local-name()
return
element water {
  element {$source/local-name()} {data($source/@name)},
  if (not($name="none"))then
  element streamsTo {
    attribute {$name} {data($to/@name)}
  }
  else ()
}


As a side-question: I want to extend the query to make it recursive: 
river "Bahr el-Djebel" streams into river "White Nile" streams into 
river "Nile" streams into sea "Mediterranean Sea"
I think I can find out how to do that, but how can I optimize the 
recursion process? Would a recursive function be efficient?

Thanks,

Paul