[hibernate-dev] Question regarding DISTINCT

[Michael Barker]

Thanks Steve,

Could you point me to the JIRA tasks, I would be interesting in seeing 
what the ultimate solution looks like?

The thing that confuses me is that distinct generally doesn't come for 
free (generally resuls in a sort by the DB).  If the result set is going 
to be the same anyway, isn't there an advantage to excluding the 
distinct clause, or is it simply not possible to determine which queries 
will benefit from it and which won't?

Regards,
Michael Barker.

Steve Ebersole wrote:
> The in-memory distincting is actually a temporary solution.  There are 
> a few JIRA cases making up the ultimate solution.
>
> But specifically to your questions, the DISTINCT applied to the query 
> is intended to limit the data transferred by the JDBC driver from 
> server to client, especially on large result sets or across poor 
> networks.  No, this is not controllable aside from using a custom 
> query translator.
>
> Michael Barker wrote:
>> Hi,
>>
>> When querying for a list of objects using ejb-ql, the keyword 
>> 'distinct' is used to de-duplicate the result set and give a correct 
>> list of objects.  However I have noticed that everytime distinct is 
>> specifed in the ejb-ql query it also gets specified in the sql 
>> query.  I am curious as the reasoning behind this.  I understand that 
>> distinct is used to remove duplicates from the SQL result set, but 
>> from what I can see, for the majority of simple use cases the sql 
>> distinct is superflous.  Running the hibernate generated query 
>> without the distinct returns the same results.  I assume that the 
>> distinct is in there for are reason, are there some more complicated 
>> use cases for which the distinct in necessary?
>>
>> My main concern is performance, is there anyway of performing an 
>> ejb-ql "select distinct..." without hibernate issuing an sql "select 
>> distinct...".
>>
>> Regards,
>> Michael Barker.
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