[GitHub] [arrow-datafusion] alamb commented on a diff in pull request #3669: Simplification Rules for Modulo Operator

[GitBox <gi...@apache.org>]

alamb commented on code in PR #3669:
URL: https://github.com/apache/arrow-datafusion/pull/3669#discussion_r985226627


##########
datafusion/optimizer/src/simplify_expressions.rs:
##########
@@ -790,6 +791,37 @@ impl<'a, S: SimplifyInfo> ExprRewriter for Simplifier<'a, S> {
                 right,
             } if !info.nullable(&left)? && left == right => lit(1),
 
+            //
+            // Rules for Modulo
+            //
+
+            // A % null --> null
+            BinaryExpr {
+                left: _,
+                op: Modulo,
+                right,
+            } if is_null(&right) => *right,
+            // null % A --> null
+            BinaryExpr {
+                left,
+                op: Modulo,
+                right: _,
+            } if is_null(&left) => *left,
+            // A % 1 --> 0
+            BinaryExpr {
+                left,
+                op: Modulo,
+                right,
+            } if !info.nullable(&left)? && is_one(&right) => lit(0),
+            // A % 0 --> DivideByZero Error
+            BinaryExpr {
+                left,
+                op: Modulo,
+                right,
+            } if !info.nullable(&left)? && is_zero(&right) => {
+                return Err(DataFusionError::ArrowError(ArrowError::DivideByZero))

Review Comment:
   I suggest not erroring in this case. It might end up causing a an expression like
   
   ```sql
   select 
     CASE 
       WHEN 0 = 0 THEN 5 
       WHEN 0 != 0 THEN 5 % 0
      END
      ...
   ```
   
   In other words, it could generate an error that would not ever actually be evaluated at runtime



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