add some notes

fe/fe-core/src/main/java/org/apache/doris/analysis/FunctionCallExpr.java

@@ -1618,6 +1618,11 @@ && collectChildReturnTypes()[0].isDecimalV3()) {
             // now first find table function in table function sets
             if (isTableFnCall) {
                 Type[] childTypes = collectChildReturnTypes();
+                // when we call explode<Array<Decimal>> with nested decimal has specific precision and scale,
+                // collectChildReturnTypes will return specific precision and scale decimal type witch may not match
+                // builtln func we defined in fe code, because we make array_support_type is actual origin type.here we
+                // temp write this if to get matched explode function and then set actually decimal type from sql to
+                // func return type. if we switch nereid would hasn't this problems.
                 if (fnName.getFunction().equalsIgnoreCase("explode") && childTypes[0].isArrayType()) {
                     // get origin type to match builtln func
                     Type[] matchFuncChildTypes = getActualArgTypes(childTypes);