At first this seemed like a bug but profiling it makes more sense ...
xdmp:random() is not a typcical xquery function as it produces a different
value every call.
( unlike most xquery functions ... looking for the word "imdopedant" but cant
find it ).
Profiling these 2 expressions on my own DB which has /*:twitter elements
for $i in (1 to 10)
let $idx := xdmp:random(1000)+1
return count((//*:twitter)[$idx])
vs
for $i in (1 to 10)
return count((//*:twitter)[xdmp:random(1000)+1])
I get similar results as you.
But the profile shows all
#1
Profile 52 ExpressionsPT6.13123S
Module:Line No.:Col No.
Count
Shallow %
Shallow µs
Deep %
Deep µs
Expression
.main:3:16
10
91
5580242
91
5580242
fn:collection()/descendant::*:twitter
.main:3:16
10
9
549566
100
6129808
(fn:collection()/descendant::*:twitter)[$idx]
.main:3:7
10
0.013
795
100
6130603
fn:count((fn:collection()/descendant::*:twitter)[$idx])
.main:1:0
1
0.0022
134
100
6130892
for $i in 1 to 10 let $idx := xdmp:random(1000) + 1 return
fn:count((fn:collection()/descendant::*:twitter)[$idx])
.main:2:12
10
0.0015
89
0.0015
89
xdmp:random(1000)
.main:2:29
10
0.001
63
0.0025
152
xdmp:random(1000) + 1
.main:1:13
1
0.000049
3
0.000049
3
1 to 10
#2
Profile 2134232 ExpressionsPT8.293776S
Module:Line No.:Col No.
Count
Shallow %
Shallow µs
Deep %
Deep µs
Expression
.main:3:16
10
68
5680234
68
5680234
fn:collection()/descendant::*:twitter
.main:3:7
10
13
1109878
100
8290044
fn:count((fn:collection()/descendant::*:twitter)[xdmp:random(1000) + 1])
.main:3:27
1067100
8.1
668253
8.1
668253
xdmp:random(1000)
.main:3:44
1067100
5.3
438670
13
1106923
xdmp:random(1000) + 1
.main:3:16
10
4.7
393009
79
6524821
(fn:collection()/descendant::*:twitter)[xdmp:random(1000) + 1]
.main:1:0
1
0.0012
101
100
8290147
for $i in 1 to 10 return
fn:count((fn:collection()/descendant::*:twitter)[xdmp:random(1000) + 1])
.main:1:13
1
0.000024
2
0.000024
2
1 to 10
----------------
So in the first case, xdmp:random gets called 10 times, and its value is used
to select a single element off that position.
In the second case xdmp:random gets called 1067100 times (10x# of docs)
and the count is the number of times xdmp:random returned *the same value* %
1000 as the position of the document.
The above example is equivilent to :
for $i in (1 to 10)
return count((//*:twitter)[position() = xdmp:random(1000)+1])
which amounts to this really
for $i in ( 1 to 10 )
return count(
for $t at $pos in //*:twitter
$pos eq xdmp:random(1000) + 1
)
So this count is returning the number of times xdmp:random(1000) + 1 returns
the same value as the position
-----------------------------------------------------------------------------
David Lee
Lead Engineer
MarkLogic Corporation
[email protected]
Phone: +1 812-482-5224
Cell: +1 812-630-7622
www.marklogic.com
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Will Thompson
Sent: Wednesday, October 03, 2012 3:19 PM
To: MarkLogic Developer Discussion
Subject: [MarkLogic Dev General] Unexpected behavior for xdmp:random() in
predicate
When I use xdmp:random() like this, I expect this to always return 1, but
that's not the result.
for $i in (1 to 10)
return count((//element)[xdmp:random(1000)+1])
=> 0 1 0 0 1 2 0 2 0 1
But when I assign the random value to a variable first, the output is as
expected and execution time is much faster.
for $i in (1 to 10)
let $idx := xdmp:random(1000)+1
return count((//element)[$idx])
=> 1 1 1 1 1 1 1 1 1 1
Is there an explanation for this?
Thanks,
Will
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