[hibernate-dev] SQM and cast targets

[Steve Ebersole]

You mean something like `treat( cast(x as some_db_type) as String)`?


On Wed, May 31, 2017 at 1:12 AM Christian Beikov <christian.beikov at gmail.com>
wrote:

> 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>
> 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> 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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>>> > hibernate-dev at lists.jboss.org
>>> > https://lists.jboss.org/mailman/listinfo/hibernate-dev
>>>
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>>
>