[hibernate-dev] CriteriaBuilder, aggregation and types

[Steve Ebersole]

Let's discuss on Hip Chat in a few after I have woken up and had some
coffee :)


On Mon, Oct 15, 2018, 6:02 AM Guillaume Smet <guillaume.smet at gmail.com>
wrote:

> Hi,
>
> We discussed a bit more with Fabio on Friday and we ended up discovering
> that we have an issue with most Expressions that contain nested
> Expressions.
>
> The Renderable contract is defined with these 3 methods:
>     String render(RenderingContext renderingContext);
>
>     default String renderProjection(RenderingContext renderingContext) {
>         return render( renderingContext );
>     }
>
>     default String renderGroupBy(RenderingContext renderingContext) {
>         return render( renderingContext );
>     }
>
> The issue is that most of the Expressions containting nested Expressions
> only implement render().
>
> This means that you basically do the following:
> call renderProjection() -> expression1 only implement render() -> we call
> render() on the nested expressions instead of renderProjection().
>
> One possible workaround is to do what was done in SearchedCaseExpression
> for all the Expressions containing nested expressions (e.g. implement the 3
> methods and have a private method taking a lambda to propagate the
> variation).
>
> You can see an example of this change here:
>
> https://github.com/hibernate/hibernate-orm/pull/2568/files#diff-8fcc837a76b1af31f11e16a01332fd1dR96
> .
>
> Note that it has to be done for *all* the Expressions containing nested
> expressions. Either that or we simplify the Renderable contract to have
> only one render() method and a parameter defining the context. That would
> allow to avoid all these changes.
>
> Thoughts?
>
> --
> Guillaume
>
> On Thu, Oct 11, 2018 at 4:01 PM Guillaume Smet <guillaume.smet at gmail.com>
> wrote:
>
> > Hi,
> >
> > We have an interesting test case here
> > https://hibernate.atlassian.net/browse/HHH-13001 about the use of
> > LiteralHandlingMode with the criteria builder. It turns out that the
> > problem is a bit more general than that.
> >
> > Let's take this (not so useful) example:
> > query.multiselect(
> >                         document.get( "id" ),
> >                         cb.sum( cb.parameter( Long.class ) ) )
> >                 .groupBy( document.get( "id" ) );
> >
> > It will lead to a NPE when trying to determine the return type of the sum
> > in:
> >
> >
> https://github.com/hibernate/hibernate-orm/blob/master/hibernate-core/src/main/java/org/hibernate/dialect/function/StandardAnsiSqlAggregationFunctions.java#L157
> >
> > The fact is that we totally lose the type information along the way.
> >
> > I don't know if it's something addressed in 6 and if we should try to fix
> > it in 5.4. In any case, I think having a workaround would be nice.
> >
> > There are a few paths we could follow:
> > 1/ at least throw a more meaningful error and provide a workaround. I
> > thought about forcing the use of the cast function but it doesn't work as
> > we have an optimization not adding the cast function is the type is
> > corresponding (see
> >
> https://github.com/hibernate/hibernate-orm/blob/master/hibernate-core/src/main/java/org/hibernate/query/criteria/internal/expression/ExpressionImpl.java#L35
> ).
> > If we remove this one and provide a helpful error message, that could
> help.
> > Note that I don't know if it's just an optimization or something mandated
> > by the spec itself.
> >
> > 2/ we could try to add a cast automatically for these functions needing a
> > proper type only when strictly necessary e.g. when you have a parameter
> (or
> > a LiteralExpression transformed to a parameter). That would mean adding a
> > method ExpressionImplementor (and change all our code to use
> > ExpressionImplementor as it currently uses the JPA version mostly
> > everywhere). That would be more transparent for the user but clearly a
> > significant amount of (dumb) work/changes. And probably a nightmare to
> > merge in 6 if this area has changed.
> >
> > Thoughts?
> >
> > --
> > Guillaume
> >
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