dawidwys commented on a change in pull request #11280: [FLINK-16377][table] 
Support inline user defined functions in expression dsl
URL: https://github.com/apache/flink/pull/11280#discussion_r397676230
 
 

 ##########
 File path: 
flink-table/flink-table-api-java/src/main/java/org/apache/flink/table/expressions/ApiExpressionUtils.java
 ##########
 @@ -198,4 +204,48 @@ public static boolean isFunction(Expression expression, 
BuiltInFunctionDefinitio
                }
                return false;
        }
+
+       /**
+        * Extracts a {@link FunctionIdentifier} for the given {@link 
CallExpression}. If the call is an inline funcion
+        * ({@link CallExpression#getFunctionIdentifier()} returns empty)
+        * <ul>
+        *     <li>it uses {@link BuiltInFunctionDefinition#getName()} ()} for 
built in functions</li>
+        *     <li>it uses {@link UserDefinedFunction#functionIdentifier()} for 
user defined functions</li>
+        *     <li>it uses {@link FunctionDefinition#toString()} ()} for any 
other functions</li>
+        * </ul>
+        */
+       public static FunctionIdentifier getFunctionIdentifier(CallExpression 
callExpression) {
+               if (callExpression.getFunctionIdentifier().isPresent()) {
+                       return callExpression.getFunctionIdentifier().get();
+               } else {
+                       return getInlineFunctionIdentifier(callExpression);
+               }
+       }
+
+       private static FunctionIdentifier 
getInlineFunctionIdentifier(CallExpression callExpression) {
+               FunctionDefinition functionDefinition = 
callExpression.getFunctionDefinition();
+               if (functionDefinition instanceof BuiltInFunctionDefinition) {
+                       return 
FunctionIdentifier.of(((BuiltInFunctionDefinition) 
functionDefinition).getName());
 
 Review comment:
   Not sure, theoretically `BuiltInFunctionDefinition` can also be registered 
in an arbitrary path. Then having a `FunctionIdentifier` in the function 
definition would be wrong. `FunctionDefinition` is not aware of the path it 
comes from. It can even be registered at multiple locations.
   
   As far as I understand the difference between built-in and user defined 
functions is where the implementation comes from. For user defined functions 
the implementation comes from the definition whereas for bult-in it is looked 
up in the planner.

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