MDEV-10844: EXPLAIN FORMAT=JSON doesn't show order direction for filesort

Currently explain format=json does not show the order direction of fields used during filesort.
This patch would remove this limitation

mysql-test/r/explain_json.result

@@ -1582,3 +1582,63 @@ EXPLAIN
   }
 }
 drop table t0,t1;
+#
+# MDEV-10844: EXPLAIN FORMAT=JSON doesn't show order direction for filesort
+#
+create table t1 (a int, b int);
+insert into t1 values (1,2),(3,4),(2,3);
+explain format=json select * from t1 order by a, b desc;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "read_sorted_file": {
+      "filesort": {
+        "sort_key": "t1.a, t1.b desc",
+        "table": {
+          "table_name": "t1",
+          "access_type": "ALL",
+          "rows": 3,
+          "filtered": 100
+        }
+      }
+    }
+  }
+}
+explain format=json select * from t1 order by a desc, b desc;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "read_sorted_file": {
+      "filesort": {
+        "sort_key": "t1.a desc, t1.b desc",
+        "table": {
+          "table_name": "t1",
+          "access_type": "ALL",
+          "rows": 3,
+          "filtered": 100
+        }
+      }
+    }
+  }
+}
+explain format=json select * from t1 order by a desc, b ;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "read_sorted_file": {
+      "filesort": {
+        "sort_key": "t1.a desc, t1.b",
+        "table": {
+          "table_name": "t1",
+          "access_type": "ALL",
+          "rows": 3,
+          "filtered": 100
+        }
+      }
+    }
+  }
+}
+drop table t1;

mysql-test/r/win.result

@@ -1779,7 +1779,7 @@ EXPLAIN
   "query_block": {
     "select_id": 1,
     "filesort": {
-      "sort_key": "row_number() over ( order by t1.s1,t1.s2)",
+      "sort_key": "row_number() over ( order by t1.s1,t1.s2) desc",
       "window_functions_computation": {
         "sorts": {
           "filesort": {

mysql-test/t/explain_json.test

@@ -406,3 +406,14 @@ explain format=json
 select a, (select max(a) from t1 where t0.a<5 and t1.b<t0.a) from t0;
 drop table t0,t1;
 
+--echo #
+--echo # MDEV-10844: EXPLAIN FORMAT=JSON doesn't show order direction for filesort
+--echo #
+
+create table t1 (a int, b int);
+insert into t1 values (1,2),(3,4),(2,3);
+explain format=json select * from t1 order by a, b desc;
+explain format=json select * from t1 order by a desc, b desc;
+explain format=json select * from t1 order by a desc, b ;
+drop table t1;
+

sql/sql_explain.cc

@@ -927,6 +927,7 @@ Explain_aggr_filesort::Explain_aggr_filesort(MEM_ROOT *mem_root,
   for (ORDER *ord= filesort->order; ord; ord= ord->next)
   {
     sort_items.push_back(ord->item[0], mem_root);
+    sort_directions.push_back(&ord->direction, mem_root);
   }
   filesort->tracker= &tracker;
 }
@@ -940,10 +941,13 @@ void Explain_aggr_filesort::print_json_members(Json_writer *writer,
   str.length(0);
   
   List_iterator_fast<Item> it(sort_items);
-  Item *item;
+  List_iterator_fast<ORDER::enum_order> it_dir(sort_directions);
+  Item* item;
+  ORDER::enum_order *direction;
   bool first= true;
   while ((item= it++))
   {
+    direction= it_dir++;
     if (first)
       first= false;
     else
@@ -951,6 +955,8 @@ void Explain_aggr_filesort::print_json_members(Json_writer *writer,
       str.append(", ");
     }
     append_item_to_str(&str, item);
+    if (*direction == ORDER::ORDER_DESC)
+      str.append(" desc");
   }
 
   writer->add_member("sort_key").add_str(str.c_ptr_safe());

sql/sql_explain.h

@@ -282,6 +282,7 @@ class Explain_aggr_node : public Sql_alloc
 class Explain_aggr_filesort : public Explain_aggr_node
 {
   List<Item> sort_items;
+  List<ORDER::enum_order> sort_directions;
 public:
   enum_explain_aggr_node_type get_type() { return AGGR_OP_FILESORT; }
   Filesort_tracker tracker;