On 04/04/2011 10:01 AM, Sergiu Dumitriu wrote:
> On 04/02/2011 02:22 PM, Caleb James DeLisle wrote:
>> After searching through documentation on JPQL (JPA's query language) I was
>> unable to find any
>> example of the "doc.object(XWiki.XWikiUsers)" construct. This means XWQL is
>> it's own standard and
>> there is no authoritative reference on it. What makes an implementation
>> compliant? I have found that
>> most HQL queries can be executed as XWQL queries with little or no
>> modification so if compliance is
>> defined as being "just like the reference implementation" then nearly all
>> HQL must be implemented in
>> order to be compliant.
>
> The goal of XWQL was to not be bound to a certain query language, but to
> be able to map it to as many QLs as possible, be they SQL-related, like
> HQL or JPQL, or other types of queries, like QBE, XPath, SPARQL. So, it
> wasn't meant from the start to be compatible with any standard.
The problem now is we don't have any specification to tell us what is valid and
what is not.
Is this a valid XWQL query?
$services.query.xwql("from BaseObject as obj where doc.fullName = obj.name and
obj.className =
'XWiki.XWikiUsers'").execute()
Run it and you might be surprised.
Based on that, we have no way of ensuring that a query which works now will
work in a new XWQL
implementation which defeats the purpose of abstracting the user away from HQL.
>
> Now, I'm not sure if the right thing to do is to move to a standard
> query language, or to stick with our own.
If we're going to define our own query language (I think there are enough
already) there are certain
things we have to do such as writing a specification. I frankly find this thing
embarrassing.
>
> - Is there any tool that allows mapping a JPQL or JDOQL query into other
> query languages?
http://www.datanucleus.org/products/accessplatform_3_0/datastores.html
These folks are mapping JDOQL and JPQL into a whole bunch of different types of
storage.
> - Is there a way to parse a query into a tree/AST?
> - Other than the fact that it's a non-standard language (and all the
> consequences of this, like no support from tools and libraries), are
> there any downsides to having our own query language?
This particular one has 2 downsides:
1. There is no official specification.
2. HQL can be run as shown above.
The major downside of implementing one correctly is that it is massively
complicated.
Caleb
>
> The benefit of XWQL was that it allowed to write domain specific
> queries, which are shorter and easier to understand (at least in theory).
>
>> Looking at the specifications I have rewritten the example query in
>> compliant JPQL and JDOQL.
>> I wrote these so that they would work if all objects were custom mapped
>> which is similar to the
>> appearance XWQL gives.
>>
>> XWQL:
>> (SELECT doc.fullName FROM XWikiDocument as doc) where doc.author =
>> 'XWiki.LudovicDubost' and
>> doc.object(XWiki.XWikiUsers).email like '%xwiki.com'
>>
>> JPQL:
>> SELECT doc.fullName FROM XWikiDocument as doc, IN(doc.xObjects) obj WHERE
>> obj.className =
>> 'XWiki.XWikiUsers' and obj.email LIKE '%xwiki.com'
>>
>> JDOQL:
>> SELECT this.fullName FROM XWikiDocument WHERE
>> this.xObjects.containsValue(obj)&& obj.className ==
>> "XWiki.XWikiUsers"&& obj.email.startsWith("xwiki.com")
>>
>>
>> I understood that XWQL was simply a translation scheme which made it appear
>> that we were using JPQL
>> with the schema we wanted when really we were using HQL with the schema we
>> had. Given that it is not
>> compliant JPQL that is not the case.
>>
>> I think when we update the schema, we should cut our losses with this thing
>> and move to something
>> which has a reference document and is more widely used.
>>
>> WDYT?
>>
>> Caleb
>>
>>
>>
>> The JPQL specification (originally called EJBQL):
>> ejb-3_0-fr-spec-ejbcore.pdf chapter 9.
>> http://jcp.org/aboutJava/communityprocess/final/jsr220/
>>
>> The JDOQL specification:
>> jdo-3_0-mrel3-spec.pdf chapter 26.
>> http://db.apache.org/jdo/specifications.html
>>
>> Easy to read, example rich descriptions:
>> http://www.datanucleus.org/products/accessplatform_3_0/jpa/jpql.html
>> http://www.datanucleus.org/products/accessplatform_3_0/jdo/jdoql.html
>
>
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