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Date: Fri, 20 Apr 2007 00:56:15 -0700 (PDT)
From: "Mamta A. Satoor (JIRA)" <jira@apache.org>
To: derby-dev@db.apache.org
Subject: [jira] Created: (DERBY-2569) The check to see if two
 DataTypeDescriptors(DTDs) are comparable or not needs to consider collation
 into decision.
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The check to see if two DataTypeDescriptors(DTDs) are comparable or not needs to consider collation into decision.
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                 Key: DERBY-2569
                 URL: https://issues.apache.org/jira/browse/DERBY-2569
             Project: Derby
          Issue Type: New Feature
          Components: SQL
    Affects Versions: 10.3.0.0
            Reporter: Mamta A. Satoor


Currently, when Derby needs to check if 2 DTDs are comparable or not, it calls TypeCompiler.comparable method. This method now has to move to DTD because we should consider collation information while deciding if 2 DTDs are comparable or not. That collation information is not available to TypeCompiler but it is available at DTD level. More information on this can be found at http://www.nabble.com/Re%3A-Changes-to-comparable-method-in-TypeCompiler-tf3603373.html#a10089238. Some of the important info from the thread is copied below

> 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. 


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