Address comments

- use OperandTypes.CHARACTER consistenly
- use OperandTypes.VARIADIC for append function
- add safeguard for atan rewrite

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

core/src/main/java/org/opensearch/sql/calcite/utils/PPLOperandTypes.java

@@ -75,8 +75,8 @@ private PPLOperandTypes() {}
 
   public static final UDFOperandMetadata NUMERIC_STRING_OR_STRING_STRING =
       UDFOperandMetadata.wrap(
-          (OperandTypes.family(SqlTypeFamily.NUMERIC, SqlTypeFamily.STRING))
-              .or(OperandTypes.family(SqlTypeFamily.STRING, SqlTypeFamily.STRING)));
+          (OperandTypes.family(SqlTypeFamily.NUMERIC, SqlTypeFamily.CHARACTER))
+              .or(OperandTypes.family(SqlTypeFamily.CHARACTER, SqlTypeFamily.CHARACTER)));
 
   public static final UDFOperandMetadata NUMERIC_NUMERIC_OPTIONAL_NUMERIC_SYMBOL =
       UDFOperandMetadata.wrap(

core/src/main/java/org/opensearch/sql/expression/function/CollectionUDF/AppendFunctionImpl.java

@@ -47,7 +47,7 @@ public SqlReturnTypeInference getReturnTypeInference() {
 
   @Override
   public UDFOperandMetadata getOperandMetadata() {
-    return UDFOperandMetadata.wrap(OperandTypes.SAME_VARIADIC);
+    return UDFOperandMetadata.wrap(OperandTypes.VARIADIC);
   }
 
   public static class AppendImplementor implements NotNullImplementor {

core/src/main/java/org/opensearch/sql/expression/function/PPLBuiltinOperators.java

@@ -501,11 +501,13 @@ public class PPLBuiltinOperators extends ReflectiveSqlOperatorTable {
           SqlFunctionCategory.NUMERIC) {
         @Override
         public SqlNode rewriteCall(SqlValidator validator, SqlCall call) {
-          SqlOperator op =
-              call.getOperandList().size() == 2
-                  ? SqlStdOperatorTable.ATAN2
-                  : SqlStdOperatorTable.ATAN;
-          ((SqlBasicCall) call).setOperator(op);
+          if (call instanceof SqlBasicCall) {
+            SqlOperator op =
+                call.getOperandList().size() == 2
+                    ? SqlStdOperatorTable.ATAN2
+                    : SqlStdOperatorTable.ATAN;
+            ((SqlBasicCall) call).setOperator(op);
+          }
           return call;
         }
       };