Re: Error in DatabaseMetaData.getColumns() with Views

Kris Jurka wrote:

>On Fri, 2 Jul 2004, Dario Fassi wrote:
>
>
>
>>Hi, I wish to report a erroneous information returned by
>>DatabaseMetaData.getColumns() method.
>>
>>This happens with ResultSet's column (7) COLUMN_SIZE and (9)
>>DECIMAL_DIGITS ,
>>when DatabaseMetaData.getColumns() inform about a VIEW with columns
>>formed with :
>>coalesce , case or numeric operations over DECIMAL fields.
>>
>>Suppose
>>
>>CREATE TABLE A ( f1 DEC(6,3), f2 dec(6,3) );
>>CREATE VIEW B as ( select ( f1 + f2 ) as f from a;
>>
>>Then DatabaseMetaData.getColumns() returns:
>>
>>VIEW B
>> F NUMERIC( 65535 , -65531 )
>>
>>
>>
>
>The problem is that f1+f2 does not retain the numeric(6,3) size
>restriction, but turns into an unbounded plain "numeric" data type. So
>when retrieving this data the precision/scale are unavailable and the
>unreasonable values you see are returned. We could return NULL instead,
>but I'm not sure that would be much more helpful if the client is
>expecting real values. Any other ideas?
>
>Kris Jurka
>
>
Yes, a few.

In the tool named PgManage (come with the commercial version of Mammoth)
, the information is accurate values for this MetaData, and I believe,
they get that values from pqsql catalog tables and not from
DatabaseMetaData interface.

More even, the engine resolve the View properly and return data values
properly typed (engine return 999.999 for f1+f2 and 999.999999 for
f1*f2 , like is expected).
So, the information if know or derived en some way for the engine ,
and/or is contained some where in catalog's tables.

I'm working in CodeGeneration tools for many DB engines (DB2, Oracle,
MS-Sql, PostgreSql, etc) , and it's impossible not to use generic
DatabaseMetaData interface to obtain metadata information. So this is a
very important problem for me.

DB2 for example do, data type escalation based on "Error propagation
Theory " , that has rules (I'm not and expert in the field) like :

[dec(6,a) + dec(6,b) ] -> [ dec( 6, max(a,b) ) ]
[ coalesce( dec(6,a) , dec(6,b) ] -> [ dec[ 6, max(a,b)] ]
[ case( dec(6,a) , dec(6,b), dec(6,c), dec(6,d) ] -> [ dec[ 6,
max(a,b,c,d)] ]
[dec(6,a) * dec(6,b ) -> [ dec( 6,a+b ) ]
etc.

This rules are taken into account in the engine data formation process,
but there are stored some where ???

Thanks for your answer.
Dario V. Fassi