Better comments in Item_in_subselect::inject_in_to_exists_cond()

sql/item_subselect.cc

@@ -2757,6 +2757,8 @@ bool Item_in_subselect::inject_in_to_exists_cond(JOIN *join_arg)
     }
 
     where_item= and_items(thd, join_arg->conds, where_item);
+
+    /* This is the fix_fields() call mentioned in the comment above */
     if (!where_item->fixed && where_item->fix_fields(thd, 0))
       DBUG_RETURN(true);
     // TIMOUR TODO: call optimize_cond() for the new where clause
@@ -2767,7 +2769,10 @@ bool Item_in_subselect::inject_in_to_exists_cond(JOIN *join_arg)
     /* Attach back the list of multiple equalities to the new top-level AND. */
     if (and_args && join_arg->cond_equal)
     {
-      /* The argument list of the top-level AND may change after fix fields. */
+      /*
+        The fix_fields() call above may have changed the argument list, so
+        fetch it again:
+      */
       and_args= ((Item_cond*) join_arg->conds)->argument_list();
       ((Item_cond_and *) (join_arg->conds))->m_cond_equal=
                                              *join_arg->cond_equal;