[hibernate-dev] Continue "parameter list" support?

Steve Ebersole steve at hibernate.org
Sat Sep 10 10:10:18 EDT 2016

WRT the "double caches"... yes there will be a few caches.

I'm not understanding what that has to do with defining a clear semantic
for a particular language element.


Look, to me this whole thing boils down to the fact that JPA defines
specific/limited support for this "collection valued input parameter"
language element.  It does that btw because the ideal in language design is
to be able to wholly understand the semantic of a part of a query based
only on the syntax[1].  Further, JPA requires that we handle a user binding
a  "collection valued input parameter" directly through #setParameter.

So all told we have a set of overloaded methods (#setParameterList) that
are not really providing any value[2], but that are needed because that
provides the only indication we ever get that a particular parameter
placeholder identifies a "collection valued input parameter" rather
than a "single
valued input parameter".  For all the reasons discussed[1], thats not the
ideal language design because it is not deterministic from the grammar.

Think of it this way...  It would be nice++ to be able to say "this
parameter is allowing multi-valued input" or "this parameter is expecting
single-valued input (multi-value input disallowed)", right?  Its the whole
reason JPA defines its grammar (BNF) the way it does, so that we can
properly validate the incoming parameter input.  Today I can do something
like this:

s.createQuery( "from Zoo z where z.name = :name" )
      .setParameterList( "name", new String[] { "San Antonio Zoo",
"Waco Zoo" } )

That is a completely "valid" query.  Its not going to return anything
useful, but syntactically it is valid because we cannot understand the
proper semantic.   And we do not understand the proper semantic because it
is not deterministic wrt the grammar/syntax.  Ideally you'd want to throw
an exception back to the user here as soon as they try to call
#setParameterList for a parameter we conceptually *know* should not accept
"collection valued input parameters"


[1] This gets into language and grammar design, but there are a few reasons
you want to encode as much semantic understanding into the syntax as
possible.  There are a few reasons for this; the 2 main ones for us:

   1. validation - again, here if I understand that a parameter placeholder
   should or should-not allow "collection valued input parameter" I can
   apply better validation up front (the sooner this type of feedback can
   happen the better)
   2. performance - anytime the query itself is not clear as to its
   semantic there is going to be a performance aspect to resolving that

[2] I can see the argument of #setParameterList providing a better API for
passing the multi-value bind value in terms of type signature.  But
ultimately IMO both  "collection valued input parameter" and  "single
valued input parameter" ought to be stored in one structure.  Today they
are not, can't because we need to know that #setParameterList was called
instead of #setParameter because that is our only understanding of the
semantic - that this is a "collection valued input parameter", not a "single
valued input parameter".

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