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From "Andrew McIntyre (JIRA)" <derby-...@db.apache.org>
Subject [jira] Commented: (DERBY-1139) Division operator may give wrong results with NUMERIC operands
Date Fri, 31 Mar 2006 23:57:40 GMT
    [ http://issues.apache.org/jira/browse/DERBY-1139?page=comments#action_12372742 ] 

Andrew McIntyre commented on DERBY-1139:
----------------------------------------

Satheesh, based on the comment shouldn't this be changed to: 

val = Math.max(NumberDataValue.MAX_DECIMAL_PRECISION_SCALE - lprec + lscale - rscale, 4);

FWIW, the algorithm that DB2 uses is described here: 

http://publib.boulder.ibm.com/infocenter/dzichelp/v2r2/index.jsp?topic=/com.ibm.db2.doc.sqlref/bjnrmstr156.htm

> Division operator may give wrong results with NUMERIC operands
> --------------------------------------------------------------
>
>          Key: DERBY-1139
>          URL: http://issues.apache.org/jira/browse/DERBY-1139
>      Project: Derby
>         Type: Bug
>   Components: SQL
>     Versions: 10.1.2.1
>     Reporter: Matthias Ohlemeyer
>     Priority: Critical

>
> The division operator '/' may give wrong results when used with NUMRERIC operands.
> Example (copied from ij):
> CREATE TABLE t (d1 DOUBLE, d2 DOUBLE, n1 NUMERIC(31,11), n2
> NUMERIC(31,11));
> INSERT INTO t VALUES (1.5, 2.5, 1.5, 2.5);
> SELECT d1/d2, n1/n2, n1*(1.0/n2) FROM t;
> 1           |2                 |3
> ----------------------------------------------------------------
> 0.6         |0                 |0.600000000000000000000000000000
> 1 row selected
> The result in column 2 should not be zero, but 0.6.
> It seems there is something wrong with the calculation of the scale. Hint from Satheesh
Bandaram:
> If you look at NumericTypeCompiler code, which calculates the scale and precision of
operation result type, the comments and the code doesn't seem to match. (getScale() method):
> NumericTypeCompiler.java
>         else if (TypeCompiler.DIVIDE_OP.equals(operator))
>         {
>             /*
>             ** Take max left scale + right precision - right scale + 1,
>             ** or 4, whichever is biggest
>             */
>             LanguageConnectionContext lcc = (LanguageConnectionContext)
>                 (ContextService.getContext(LanguageConnectionContext.CONTEXT_ID));
>             // Scale: 31 - left precision + left scale - right scale
>             val = Math.max(NumberDataValue.MAX_DECIMAL_PRECISION_SCALE - lprec + lscale
- rscale, 0);
>         }
> Here val is returning zero for scale, it seems.

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