[hibernate-dev] SQM and cast targets

[Christian Beikov]

So during parsing you try to lookup the castTarget and if it can't be 
found, just pass through? If you pass it through, what would be the type 
of the expression?

I'd like to present an idea I just had. How about we reuse the "TREAT" 
function/operator for doing these "casts" to named types. Applying the 
operator does not necessarily cause a SQL "cast" i.e. if the expression 
is a select item and the JDBC driver supports converting a value to the 
desired type automatically, there is no need for a cast. The main 
difference to a "cast" function would be, that the expression type will 
be set to the desired type, whereas the "cast" function will set the 
type to "unknown" i.e. requiring the user to use the treat operator 
around the cast. The cast function will then only ever be used for the 
pass-through case. Wdyt?


Mit freundlichen Grüßen,
------------------------------------------------------------------------
*Christian Beikov*
Am 30.05.2017 um 18:00 schrieb Steve Ebersole:
> How about this rule then?
>
> castTarget
> // should allow either
> // - named cast (IDENTIFIER)
> //- JavaTypeDescriptorRegistry (imported) key
> //- java.sql.Types field NAME (coded cast synonym - field's value)
> //- "pass through"
> //- coded cast (INTEGER_LITERAL)
> //- SqlTypeDescriptorRegistry key
> : IDENTIFIER | INTEGER_LITERAL
> ;
>
> On Mon, May 29, 2017 at 11:16 AM Steve Ebersole <steve at hibernate.org 
> <mailto:steve at hibernate.org>> wrote:
>
>     Yes, ultimately these need to resolve to SqlTypeDescriptor.  So
>     perhaps we allow both.
>
>     What I just want to get out of is the open-ended-ness.
>     Non-determinism is bad.  E.g., like what you just mentioned...
>      how should the parser understand that "TEXT" `cast(x as TEXT)` is
>     a database type name versus Java class name versus something
>     else?  Structurally we cannot - one String is syntactically the
>     same as any other String.
>
>     So do we just accept some policy of "well if we don't understand
>     it we'll just pass it through to the database"?  To me that's just
>     a cop-out.  Not to mention that it invariably leaves the door open
>     to non-portability.  If instead we limited this to just Java types
>     (JavaTypeDescriptorRegistry keys) and JDBC type codes
>     (SqlTypeDescriptorRegistry keys) we can fully support this in a
>     portable manner.  Now that does lead to a question for databases
>     which make the silly decision (looking at you pgsql) to map
>     multiple types to the same JDBC type code.
>
>     As much as possible I think we ought to not be relying on the
>     database to validate these kinds of things.  An error from the
>     database is going to be much less descriptive as to what exactly
>     is wrong compared to a validation done by Hibernate.
>
>     Not sure the correct answer, just some thoughts.
>
>     An option is to allow 3 types of cast targets:
>
>      1. Java type name we can resolve against the
>         JavaTypeDescriptorRegistry
>      2. A JDBC type (either by code or name) we can resolve against
>         the SqlTypeDescriptorRegistry
>      3. Any other text we can resolve against the Dialect as a "valid
>         SQL type"
>
>     I'm kind of leery of (3), but if everyone else agrees it is
>     important to allow that non-portability then I will consider it. 
>     And keep in mind that this is really only needed for databases
>     like pgsql to handle the multiple types it maps to a single JDBC
>     type code...  all other cases can (and should) be handled by (1)
>     and (2).
>
>
>
>     On Mon, May 29, 2017 at 10:36 AM Christian Beikov
>     <christian.beikov at gmail.com <mailto:christian.beikov at gmail.com>>
>     wrote:
>
>         Sounds good, although I guess there might be cases when ONLY this
>         approach won't work that well.
>
>         I am specifically thinking about casts to the various
>         character types
>         that are available in the different DBMS. A cast to "String"
>         might work
>         most of the time, but we should still have an option to cast
>         to CLOB,
>         TEXT or whatever other datatype a DBMS offers.
>
>
>         Mit freundlichen Grüßen,
>         ------------------------------------------------------------------------
>         *Christian Beikov*
>         Am 29.05.2017 um 16:17 schrieb Steve Ebersole:
>         > Currently casting in HQL is under-defined and open-ended
>         (and therefore
>         > pretty inconsistent).  What does that mean?  Well, what is a
>         valid cast
>         > target in HQL?  There really is not a defined
>         > answer to that.
>         >
>         > I'd like to start formalizing the answer to this.
>         >
>         > Specifically, I am thinking this should be defined around
>         > JavaTypeDescriptor.  So that we'd understand any Java type
>         registered with
>         > with JavaTypeDescriptorRegistry, and specifically any that
>         properly
>         > implements `#getJdbcRecommendedSqlType` (using the Dialect
>         to resolve the
>         > cast target in the generated SQL).
>         >
>         > Anyone have objections to this?  Thoughts?
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