Another try at making numeric MODULO operator produce the right answer.

Although it was now using the right equation, it was making bogus choices
of the precision to compute intermediate results to.  I'm not sure this
is really right even yet, but it's better than before ...

src/backend/utils/adt/numeric.c

@@ -5,7 +5,7 @@
  *
  *	1998 Jan Wieck
  *
- * $Header: /cvsroot/pgsql/src/backend/utils/adt/numeric.c,v 1.39 2001/03/22 06:16:17 momjian Exp $
+ * $Header: /cvsroot/pgsql/src/backend/utils/adt/numeric.c,v 1.40 2001/04/14 02:10:57 tgl Exp $
  *
  * ----------
  */
@@ -1134,7 +1134,6 @@ numeric_mod(PG_FUNCTION_ARGS)
 
 	mod_var(&arg1, &arg2, &result);
 
-	result.dscale = result.rscale;
 	res = make_result(&result);
 
 	free_var(&result);
@@ -3281,29 +3280,42 @@ mod_var(NumericVar *var1, NumericVar *var2, NumericVar *result)
 {
 	NumericVar	tmp;
 	int			save_global_rscale;
+	int			div_dscale;
 
 	init_var(&tmp);
 
 	/* ---------
 	 * We do this using the equation
 	 *		mod(x,y) = x - trunc(x/y)*y
-	 * We fiddle a bit with global_rscale to control result precision.
+	 * We set global_rscale the same way numeric_div and numeric_mul do
+	 * to get the right answer from the equation.  The final result,
+	 * however, need not be displayed to more precision than the inputs.
 	 * ----------
 	 */
 	save_global_rscale = global_rscale;
-	global_rscale = var2->rscale + 2;
+
+	div_dscale = MAX(var1->dscale + var2->dscale, NUMERIC_MIN_DISPLAY_SCALE);
+	div_dscale = MIN(div_dscale, NUMERIC_MAX_DISPLAY_SCALE);
+	global_rscale = MAX(var1->rscale + var2->rscale,
+						NUMERIC_MIN_RESULT_SCALE);
+	global_rscale = MAX(global_rscale, div_dscale + 4);
+	global_rscale = MIN(global_rscale, NUMERIC_MAX_RESULT_SCALE);
 
 	div_var(var1, var2, &tmp);
 
+	tmp.dscale = div_dscale;
+
 	/* do trunc() by forgetting digits to the right of the decimal point */
 	tmp.ndigits = MAX(0, MIN(tmp.ndigits, tmp.weight + 1));
-	tmp.rscale = var2->rscale;
 
-	global_rscale = var2->rscale;
+	global_rscale = var2->rscale + tmp.rscale;
+
 	mul_var(var2, &tmp, &tmp);
 
 	sub_var(var1, &tmp, result);
 
+	result->dscale = MAX(var1->dscale, var2->dscale);
+
 	global_rscale = save_global_rscale;
 	free_var(&tmp);
 }