I have found a situation where the number of times an expression is evaluated
is greatly multiplied when the variables are declared in the prolog versus
using the "let" keyword. There's something going on here that I just can't
figure out.
I know the code is a little strange, but it is the simplest example that I
could come up with that would definitely show the differences:
Using declare:
declare variable $set := (1,2,3,4,5,6,7,8,9,10);
declare variable $set-length := fn:count($set);
let $value :=
for $s at $i in 1 to 100000
return $set[$set[$i + $set-length div 2]]
return $value
Total time: 16 sec
Number of times "$set-length div 2" is evaluated: 9999990
----------------------------------------------------------------------------------
Using let:
let $set := (1,2,3,4,5,6,7,8,9,10)
let $set-length := fn:count($set)
let $value :=
for $s at $i in 1 to 100000
return $set[$set[$i + $set-length div 2]]
return $value
Total time: 0.4 sec
Number of times "$set-length div 2" is evaluated: 100000
----------------------------------------------------------------------------------
And check this out, using declare but moving the index expression out to a let:
declare variable $set := (1,2,3,4,5,6,7,8,9,10);
declare variable $set-length := fn:count($set);
let $value :=
for $s at $i in 1 to 100000
let $index := $i + $set-length div 2
return $set[$set[$index]]
return $value
Total time: 0.37 sec
Number of times "$set-length div 2" is evaluated: 100000
----------------------------------------------------------------------------------
So what's going on here? I'm using MarkLogic 5
-Ryan
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