Hi Huub,
Thank you for your reply.
I tried your suggestions, but it does not make any
difference.
I changed Query 1 to this:
(: list waters and where they stream to (if any):)
for $source in //(sea|river|lake)
let $toId := $source/to/@water
let $to := //*[@id=$toId][1]
let $name := if (empty($to)) 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 ()
}
but there is no performance gain. The query still executes
at least 10 times slower than Query 2.
Thanks for the empty($to) suggestion.
As for the recursive algorithm: in the meantime I wrote the
query for that and it works like a charm!
Paul
Hi Paul,
Op 4 aug. 2014, om 09:27 heeft Paul
Swennenhuis <
paul@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.