Hi Paul,
Op 4 aug. 2014, om 09:27 heeft Paul Swennenhuis < p...@swennenhuis.nl > het volgende geschreven:
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]
You start to search for “sea” elements at the very top of the db, then, for “river” elements you start to search at the very top of the db, then, for “lake” elements you start to search at the very top of the db. And you do this for every $source you process. This is different from "//(sea|river|lake)" where you start at the top (once) and then match sea, river or lake elements. In the second query you find all sea, river and lake elements once and then use that sequence to search in, that would be (much) faster.
It might even be faster to just search all element and filter on @id (BaseX can then use the attribute index and just needs to use it once, probably), f.i.:
let $toWaters := //*[@id = $toId]
and $toWaters contains all waters (sea, river and lake elements) the $source streams to. Add a [1] if you just need the first one like you did. (If you know that all waters mentioned in $source/to exist in your db, wouldn't it be better to restrict $toId instead of $to, i.e. just use the first $source/to element?)
let $name := if (empty($to/local-name())) then “none” else $to/local-name()
I am not sure I understand but wouldn't empty($to) do the trick?
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?
Yes, that would do the trick. Generally, tail-recursiveness is a good thing, but in this case it wouldn't matter much probably. Just watch out for those weird rivers that flow back into the lake they originate from ;-).
Regards,
Huib Verweij.
