danny0405 commented on a change in pull request #2272:
URL: https://github.com/apache/calcite/pull/2272#discussion_r528559816



##########
File path: core/src/main/java/org/apache/calcite/tools/RelBuilder.java
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@@ -495,6 +495,10 @@ public RexNode field(int inputCount, String alias, String 
fieldName) {
 
   /** Returns a reference to a given field of a record-valued expression. */
   public RexNode field(RexNode e, String name) {
+    if (!e.getType().isStruct()) {
+      throw new IllegalArgumentException("Requested field " + name + " in 
non-struct expression "
+          + e.toString() + " (type: " + e.getType() + ")");
+    }

Review comment:
       I have the impression that the `RelBuilder` build rel trees based on all 
kinds of assumptions, it assumes that the invoker knows how to behave with the 
valid operands. We may need to check all the interfaces in `RelBuilder`, this 
PR is a good start.




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