Thanks, Dan. This information was very helpful.
And thanks, Bryan, for always capturing different pieces of information from
Derby mailing list and putting it together on Derby wiki. It will surely be
helpful to others.
Mamta
On 4/19/07, Daniel John Debrunner <[EMAIL PROTECTED] > wrote:
Mamta Satoor wrote:
> I think what you are suggesting is to move comparable method out from
> the TypeCompiler and into DataTypeDescriptor altogether. So, the
> existing code, where we use TypeCompiler to decide if 2 types can be
> compared or not should now call a method on DTD to determine
> comparability. This might be cleaner than stuffing collation information
> in CharTypeCompiler but I am just wondering why was comparable not
> defined on DTD at the very start. Why do we go through TypeCompiler and
> what functionality does TypeCompiler provide that DTD does not? In other
> words, I don't understand the connection between TypeCompiler and DTD
> and how they fit together.
It's not that TypeCompiler provides functionality that DTD does not, but
instead DTD has functionality/information that TypeCompiler does not.
Ignoring the "compiler" aspect for the moment there are two components
to a DataTypeDescriptor, the underlying SQL type (INTEGER, CHAR,
VARCHAR, XML etc.) represented as TypeId and attributes of the
descriptor (nullablity, length, precision, scale and now collation).
Thus
DTD = TypeId + {attributes}
Some functionality is applicable to a type regardless of a specific
DTD's attributes, thus methods for that functionality can be declared on
TypeId instead of DTD.
Some functionality on the other hand needs the attribute information as
well, say the display length of a type is a function of its
length/precision&scale and its underlying SQL type.
The collation changes have moved the comparable check from being only
reliant on the SQL type (TypeId) to being dependent on the type's
attributes (collation type and implicit/explicit). Thus the original
location for the comparable method made sense, but now does not.
The TypeCompiler/TypeId split was due to an early plan to have a
execute-only version of the technology, this never happened as there was
no demand for it. One of the benefits of a SQL engine is the ability to
execute arbitrary queries, which would not be available in an execute
only version. Code cleanup could be done here which probably would
decrease the footrprint of derby.
HTH,
Dan.
