Hi Andrew,
I agree, every processor has its own optimizations. :)
However, in the case of ML, I would stick with your original advice: use full
paths to elements rather than //. In ML this allows the processor to use
several indexes to resolve the full path, rather than a single index.
I would also try to get your constraints into a single XPath expression rather
than in the where clause of a FLWOR.
Someone else from Mark Logic will chime in if these recommendations have
changed, but I think they are safe.
Kelly
Message: 4
Date: Tue, 24 Nov 2009 09:53:59 +0000
From: Andrew Welch <andrew.j.we...@gmail.com>
Subject: Re: [MarkLogic Dev General] XML structure/schema design for
MLS
To: General Mark Logic Developer Discussion
<general@developer.marklogic.com>
Message-ID:
<74a894af0911240153i3f48f80n2e4e7a831ef86...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
2009/11/23 Lee, David <d...@epocrates.com>:
> I think what Karl is expressing is frustration that basic xpath
> expressions appear not to use indexes.
> I too am 'in the dark' about that ... and would love some advise.
> Why, for example
> ? ? ? ?cts:search( doc("doc.xml")/FOO/BAR , "text")
>
> uses index ("instant results" )
>
> but apparently
> ? ? ? ?doc("doc.xml")/FOO[BAR eq 'text']
>
> seems to iterate through the list and not use indexes. (painfully slow
> results ... )
Quite often it's down to the processor being able to recognise what you are
trying to do and then rewrite it to be faster. In XSLT the usual advice is
specify the full path and avoid //foo because it requires walking the entire
input tree, but with a database it's better to avoid the full path and use //
instead (where the results would be the same) because indexes can be used.
I've also heard that using a flowr expression instead of single xpath can help
because it simplifies the expression which helps the processor spot what it can
optimise.
--
Andrew Welch
http://andrewjwelch.com
Kernow: http://kernowforsaxon.sf.net/
_______________________________________________
General mailing list
General@developer.marklogic.com
http://xqzone.com/mailman/listinfo/general
