On Mar 18, 2008, at 15:38 , Keulen, M. van (Maurice) wrote:
> Jan Rittinger wrote:
>>
>> On 03/18/2008 02:43 PM, Keulen, M. van (Maurice) wrote with
>> possible deletions:
>>> Hi,
>>>
>>> I know that the XQUF prohibits it, but for writing robust
>>> functions, it is often necessary to mix query and update/docmgmt
>>> functionality. Some examples:
>>
>> Hi Maurice,
>>
>> I don't think what you suggest is necessary. If you want to query
>> with side effects -- which is certainly not what XQuery was
>> initially introduced for -- and you want to simulate XQueryP then
>> you can *always* use updating queries and return your results in a
>> special result document.
> What you mean is to do the updates and instead of returning the
> query result, store it in the database as well ... and then use a
> second query to retrieve the result? That only works if I am sure
> that the first query is completed before the second one starts.
> Furthermore, an update and a docmgmt operation also do not mix, so
> you need three queries:
> • (docmgmt) one to make sure that there is a document in the
> database for the result (if not, it adds it)
> • (update) one to do the updates and insert the result in the
> result document
> • (query) one to get the result document
> A lot of overhead if you ask me.
>
> Furthermore, I access the database by sending SOAP-messages to the
> XRPC-interface ... in other words: asynchronous communication. So,
> waiting for a query to complete is hard ... and you cannot send
> different queries in one SOAP-message (only multiple calls for the
> same query, but with other parameters).
>
> Jennie: my SOAP-messages do not come from MonetDB/XQuery, but from
> outside (i.e., some scripting language).
Then, an order can only be guaranteed if you, in your scripts, send a
message, wait for result, and send the next message. In all other
cases, unfortunately, no guarantee, as an xrpc server immediately
start handling a request when it's received.
Jennie
>>>
>>> * Expression that queries/updates document, but if the
>>> document does
>>> not exist, adds an empty version of it:
>>> let $name := "foo.xml"
>>> let $emptydoc := <root/>
>>> let $doc := if (empty(pf:documents()[.=$name])) then
>>> (put($emptydoc,$name),$emptydoc) else doc("foo.xml")
>>> return $doc//bar
>>
>> Here you expect, that the document management stuff is done just in
>> time and thus with side effects! Your query fails if the document
>> management stuff is done after the query part.
> No, not true. If "foo.xml" is not in the database yet, I want to add
> it, but simply continue with $emptydoc. The query does not depend on
> the document to be added immediately.
>>> * Expression that adds to a log-file:
>>> let $log := doc("log.xml")
>>> let $expr := ...some expression...
>>> return (do insert <entry>...{$expr}...<entry> as last into
>>> $log,$expr)
>>
>> You can also return a ``nested'' sequence and query the result to
>> split up your query result and the log files. In my eyes you once
>> more try to ``cheat'' to have more than a single result stream.
> What do you mean by "nested" sequence? Due to the typing rules, an
> expression is either an updating expression (without query result)
> or not (with query result). This implies I cannot within one query
> do an update and return a result. This is exactly the thing I want
> to do. I do not want two result streams, I just want a query to have
> one query result but with side-effects to the database.
>
> Maurice.
> --
> ----------------------------------------------------------------------
> Dr.Ir. M. van Keulen - Assistant Professor, Data Management Technology
> Univ. of Twente, Dept of EEMCS, POBox 217, 7500 AE Enschede,
> Netherlands
> Email: [EMAIL PROTECTED], Phone: +31 534893688, Fax: +31
> 534892927
> Room: ZI 3039, WWW: http://www.cs.utwente.nl/~keulen
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