Hi Christian,
Just tried the query in BaseX 8.
Sorry, no improvement in performance.
Paul
Hi Paul,
thanks for your feedback. Are you working with 7.9? If it's not too
much of a hassle for you, I would be interested to hear if you get
better performance with the latest 8.0 snapshot?
Christian
[1] http://files.basex.org/releases/latest/
On Mon, Aug 4, 2014 at 11:57 AM, Paul Swennenhuis <p...@swennenhuis.nl> wrote:
Hi Christian,
Sorry, also doesn't improve performance.
I even tried to copy the optimized line for the selection, as found in the
Query Info pane:
(: list waters and where they stream to (if any):)
for $source in ((db:open-pre("facts",0)/descendant::*:sea union
db:open-pre("facts",0)/descendant::*:river union
db:open-pre("facts",0)/descendant::*: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 ()
}
No improvement.
The problem seems to be in the line that assigns the $to variable
If I reuse the main node selection there the query executes fast.
Like such:
(: list waters and where they stream to (if any):)
let $sources := /descendant::sea | /descendant::river | /descendant::lake
for $source in $sources
let $toId := $source/to/@water
let $to := $sources[@id=$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 ()
}
The original line, let $to := //*[@id=$toId][1], apparently is very
expensive.
I could do some testing with the profiling tools to see if I'm right.
Paul
Hi Paul,
//(sea|river|lake)
Due to the (somewhat peculiar) semantics of XPath, this path is identical
to...
/descendant-or-self::node()/
(child::sea | child::river | child::lake)
...and it creates a massive amount of intermediate results. You could
try to rewrite it to...
/descendant::sea | /descendant::river |
/descendant::lake
...or...
/descendant::*[local-name() = ('sea', 'river', 'lake')]
...and I will try to tweak our optimizer to automatically do this for
you in future (it already works for single steps).
Christian
