4 Apr
2011
4 Apr
'11
6:47 p.m.
Le 04/04/11 16:37, Caleb James DeLisle a écrit :
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Ludovic Dubost
Blog: http://blog.ludovic.org/
XWiki: http://www.xwiki.com
Skype: ldubost GTalk: ldubost
On 04/04/2011 10:01 AM, Sergiu Dumitriu wrote:
The key objective of XWQL is to abstract from the XWiki point of view
and make it as simple as possible to write queries.
If I take this (valid) query in XWQL:
from doc.object(Blog.BlogPostClass) as blogarticle where 'Blog.Blogging'
member of blogarticle.category
It seems to me that it would be more complex in JPQL or JDOQL, which is
not very cool.
I'm -1 from any new query language that would end up being more complex.
If I take your 2 downsides:
1/ Official spec
Well we should write one. It seems normal to me that a new query
language has no spec.
But this does not mean that we don't need a language.
Unless we can find a standard language that is as easy to use as XWQL,
let's stick to XWQL.
If we need to improve XWQL let's improve it.
2/ HQL can be run
That's an implementation issue. XWQL's objective is to write and run
after translations to whatever is needed by the backend.
It is VERY important to have a simple query language. It took a long
time to learn how to run joins in HQL which are not needed from a pure
"functionnal" standpoint.
BTW I had fixed XWiki's query generator which now generates XWQL. It
needs XWiki 2.7.1+
Example here:
http://www.ludovic.org/xwiki/bin/view/Test/Query?query=1&classname=Blog…
I will document it and publish it on extensions.xwiki.org
Ludovic
On 04/02/2011 02:22 PM, Caleb James DeLisle
wrote:
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.
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. 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