Re: [bdbxml] "Pure" XPath2 versus XQuery

Kiril,

The query optimizer works on both XPath and XQuery expressions, as
you've seen.  There are, however, some cases where it can get
tripped up by one or the other.  These cases are being addressed
with continual improvements in the optimizer.

In general, as long as the correct indexes are used, based on the
query plan output, XQuery vs XPath should perform about
the same.   You should, however, make sure that the query plans are
using expected indexes.

Using XPath when possible is a reasonable rule of thumb, as
the expressions are often more compact.   But, as John recently
pointed out, it can sometimes be better to use simpler expressions (e.g.
variables) in predicates, which may argue for XQuery.  Again, the
best results are obtained through experimentation.

It's worth mentioning that for repeated queries, a very
large benefit is realized by pre-parsing an expression into
XmlQueryExpression, and reusing that object.

Regards,

George


> Hi all,
>
> I wonder if there is performance difference (or other pros and  
> cons) when executing simple queries as XPath or XQuery.
> Example:
>
> /Item[@type='Mail' and days-from-duration(fn:current-date()-xs:date 
> (/Item/CreationTime)) <= 7]
>
> versus
>
> for $x in /Item[@type='Mail']
> where days-from-duration(fn:current-date()-xs:date($x/ 
> CreationTime)) <= 5
> return $x
>
> I examined the query plans and they both seem almost same...
> Well, this is a simple example, but should I always prefer XPath,  
> if it does the job?
>
> TIA,
>
> Kiril.
>



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