[GitHub] [drill] paul-rogers commented on a change in pull request #2006: DRILL-7615: UNION ALL query returns the wrong result for the decimal value

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paul-rogers commented on a change in pull request #2006: DRILL-7615: UNION ALL query returns the wrong result for the decimal value
URL: https://github.com/apache/drill/pull/2006#discussion_r386776641
 
 

 ##########
 File path: common/src/main/java/org/apache/drill/common/types/Types.java
 ##########
 @@ -795,15 +807,25 @@ public static boolean isSortable(MinorType type) {
    */
   public static MajorType.Builder calculateTypePrecisionAndScale(MajorType leftType, MajorType rightType, MajorType.Builder typeBuilder) {
     if (leftType.getMinorType().equals(rightType.getMinorType())) {
-      final boolean isScalarString = Types.isScalarStringType(leftType) && Types.isScalarStringType(rightType);
-      final boolean isDecimal = isDecimalType(leftType);
+      boolean isScalarString = Types.isScalarStringType(leftType) && Types.isScalarStringType(rightType);
+      boolean isDecimal = isDecimalType(leftType);
 
-      if ((isScalarString || isDecimal) && leftType.hasPrecision() && rightType.hasPrecision()) {
+      if (isScalarString && leftType.hasPrecision() && rightType.hasPrecision()) {
         typeBuilder.setPrecision(Math.max(leftType.getPrecision(), rightType.getPrecision()));
       }
 
-      if (isDecimal && leftType.hasScale() && rightType.hasScale()) {
-        typeBuilder.setScale(Math.max(leftType.getScale(), rightType.getScale()));
+      if (isDecimal) {
+        int scale = Math.max(leftType.getScale(), rightType.getScale());
+        // resulting precision should take into account resulting scale value and be calculated as
+        // sum of two components:
+        // - max integer digits number (precision - scale) for left and right;
+        // - resulting scale.
+        // So for the case of cast(9999 as decimal(4,0)) and cast(1.23 as decimal(3,2))
+        // resulting scale would be Max(0, 2) = 2 and resulting precision would be Max(4 - 0, 3 - 2) + 2 = 6.
+        // In this case, both values would fit into decimal(6, 2): 9999.00, 1.23
+        int precision = Math.max(leftType.getPrecision() - leftType.getScale(), rightType.getPrecision() - rightType.getScale()) + scale;
+        typeBuilder.setPrecision(precision);
+        typeBuilder.setScale(scale);
 
 Review comment:
   Nit: please split the long lint.
   

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