Make EXPLAIN FORMAT=JSON be able to show the key that's used for sorting.

This will be useful for window functions development.

mysql-test/r/analyze_format_json.result

@@ -490,6 +490,7 @@ ANALYZE
     "volatile parameter": "REPLACED",
     "having_condition": "(TOP > t2.a)",
     "filesort": {
+      "sort_key": "t2.a",
       "r_loops": 1,
       "volatile parameter": "REPLACED",
       "r_used_priority_queue": false,
@@ -519,6 +520,7 @@ ANALYZE
     "r_loops": 1,
     "volatile parameter": "REPLACED",
     "filesort": {
+      "sort_key": "t2.a",
       "r_loops": 1,
       "volatile parameter": "REPLACED",
       "r_used_priority_queue": false,
@@ -559,6 +561,7 @@ ANALYZE
     "r_loops": 1,
     "volatile parameter": "REPLACED",
     "filesort": {
+      "sort_key": "t2.a",
       "r_loops": 1,
       "volatile parameter": "REPLACED",
       "r_used_priority_queue": false,
@@ -680,6 +683,7 @@ ANALYZE
     "r_loops": 1,
     "volatile parameter": "REPLACED",
     "filesort": {
+      "sort_key": "group_concat(t3.f3 separator ',')",
       "r_loops": 1,
       "volatile parameter": "REPLACED",
       "r_used_priority_queue": false,
@@ -687,6 +691,7 @@ ANALYZE
       "volatile parameter": "REPLACED",
       "temporary_table": {
         "filesort": {
+          "sort_key": "(subquery#2)",
           "r_loops": 1,
           "volatile parameter": "REPLACED",
           "r_used_priority_queue": false,

mysql-test/r/analyze_stmt_orderby.result

@@ -172,6 +172,7 @@ EXPLAIN
   "query_block": {
     "select_id": 1,
     "filesort": {
+      "sort_key": "t2.b",
       "temporary_table": {
         "table": {
           "table_name": "t0",
@@ -204,6 +205,7 @@ ANALYZE
     "r_loops": 1,
     "r_total_time_ms": "REPLACED",
     "filesort": {
+      "sort_key": "t2.b",
       "r_loops": 1,
       "r_total_time_ms": "REPLACED",
       "r_limit": 4,
@@ -256,6 +258,7 @@ EXPLAIN
     "select_id": 1,
     "read_sorted_file": {
       "filesort": {
+        "sort_key": "t0.a",
         "table": {
           "table_name": "t0",
           "access_type": "ALL",
@@ -289,6 +292,7 @@ ANALYZE
     "read_sorted_file": {
       "r_rows": 10,
       "filesort": {
+        "sort_key": "t0.a",
         "r_loops": 1,
         "r_total_time_ms": "REPLACED",
         "r_used_priority_queue": false,
@@ -345,6 +349,7 @@ ANALYZE
     "r_loops": 1,
     "r_total_time_ms": "REPLACED",
     "filesort": {
+      "sort_key": "t2.c",
       "r_loops": 1,
       "r_total_time_ms": "REPLACED",
       "r_used_priority_queue": false,
@@ -454,6 +459,7 @@ ANALYZE
     "r_loops": 1,
     "r_total_time_ms": "REPLACED",
     "filesort": {
+      "sort_key": "count(distinct t5.b)",
       "r_loops": 1,
       "r_total_time_ms": "REPLACED",
       "r_limit": 1,
@@ -461,6 +467,7 @@ ANALYZE
       "r_output_rows": 2,
       "temporary_table": {
         "filesort": {
+          "sort_key": "t5.a",
           "r_loops": 1,
           "r_total_time_ms": "REPLACED",
           "r_used_priority_queue": false,
@@ -510,8 +517,10 @@ EXPLAIN
   "query_block": {
     "select_id": 1,
     "filesort": {
+      "sort_key": "count(distinct t5.b)",
       "temporary_table": {
         "filesort": {
+          "sort_key": "t5.a",
           "temporary_table": {
             "table": {
               "table_name": "t6",

mysql-test/r/explain_json.result

@@ -486,6 +486,7 @@ EXPLAIN
         "query_block": {
           "select_id": 2,
           "filesort": {
+            "sort_key": "t1.a",
             "temporary_table": {
               "table": {
                 "table_name": "t1",
@@ -529,6 +530,7 @@ EXPLAIN
         "query_block": {
           "select_id": 2,
           "filesort": {
+            "sort_key": "t1.a",
             "temporary_table": {
               "table": {
                 "table_name": "t1",
@@ -1129,6 +1131,7 @@ EXPLAIN
     "select_id": 1,
     "having_condition": "(TOP > t2.a)",
     "filesort": {
+      "sort_key": "t2.a",
       "temporary_table": {
         "table": {
           "table_name": "t2",
@@ -1147,6 +1150,7 @@ EXPLAIN
   "query_block": {
     "select_id": 1,
     "filesort": {
+      "sort_key": "t2.a",
       "temporary_table": {
         "table": {
           "table_name": "t2",
@@ -1176,6 +1180,7 @@ EXPLAIN
   "query_block": {
     "select_id": 1,
     "filesort": {
+      "sort_key": "t2.a",
       "temporary_table": {
         "table": {
           "table_name": "t2",

mysql-test/r/win.result

@@ -1436,6 +1436,7 @@ EXPLAIN
   "query_block": {
     "select_id": 1,
     "filesort": {
+      "sort_key": "t1.a",
       "window_functions_computation": {
         "temporary_table": {
           "table": {
@@ -1488,6 +1489,7 @@ EXPLAIN
     "select_id": 1,
     "having_condition": "(MX in (3,5,7))",
     "filesort": {
+      "sort_key": "t1.b",
       "window_functions_computation": {
         "temporary_table": {
           "table": {

sql/sql_explain.cc

@@ -30,6 +30,7 @@ const char * STR_IMPOSSIBLE_WHERE= "Impossible WHERE";
 const char * STR_NO_ROWS_AFTER_PRUNING= "No matching rows after partition pruning";
 
 static void write_item(Json_writer *writer, Item *item);
+static void append_item_to_str(String *out, Item *item);
 
 Explain_query::Explain_query(THD *thd_arg, MEM_ROOT *root) : 
   mem_root(root), upd_del_plan(NULL),  insert_plan(NULL),
@@ -877,8 +878,7 @@ void Explain_select::print_explain_json(Explain_query *query,
         case AGGR_OP_FILESORT:
         {
           writer->add_member("filesort").start_object();
-          if (is_analyze)
-            ((Explain_aggr_filesort*)node)->tracker->print_json_members(writer);
+          ((Explain_aggr_filesort*)node)->print_json_members(writer, is_analyze);
           break;
         }
         case AGGR_OP_REMOVE_DUPLICATES:
@@ -905,6 +905,41 @@ void Explain_select::print_explain_json(Explain_query *query,
     writer->end_object();
 }
 
+void Explain_aggr_filesort::init(THD *thd, Filesort *filesort)
+{
+  for (ORDER *ord= filesort->order; ord; ord= ord->next)
+  {
+    sort_items.push_back(ord->item[0], thd->mem_root);
+  }
+  filesort->tracker= &tracker;
+}
+
+void Explain_aggr_filesort::print_json_members(Json_writer *writer, 
+                                               bool is_analyze)
+{
+  char item_buf[256];
+  String str(item_buf, sizeof(item_buf), &my_charset_bin);
+  str.length(0);
+  
+  List_iterator_fast<Item> it(sort_items);
+  Item *item;
+  bool first= true;
+  while ((item= it++))
+  {
+    if (first)
+      first= false;
+    else
+    {
+      str.append(", ");
+    }
+    append_item_to_str(&str, item);
+  }
+
+  writer->add_member("sort_key").add_str(str.c_ptr_safe());
+
+  if (is_analyze)
+    tracker.print_json_members(writer);
+}
 
 void Explain_basic_join::print_explain_json(Explain_query *query, 
                                             Json_writer *writer, 
@@ -1222,7 +1257,7 @@ int Explain_table_access::print_explain(select_result_sink *output, uint8 explai
     extra_buf.append(STRING_WITH_LEN("Using temporary"));
   }
 
-  if (using_filesort || this->using_filesort)
+  if (using_filesort || this->pre_join_sort)
   {
     if (first)
       first= false;
@@ -1282,6 +1317,15 @@ static void write_item(Json_writer *writer, Item *item)
   writer->add_str(str.c_ptr_safe());
 }
 
+static void append_item_to_str(String *out, Item *item)
+{
+  THD *thd= current_thd;
+  ulonglong save_option_bits= thd->variables.option_bits;
+  thd->variables.option_bits &= ~OPTION_QUOTE_SHOW_CREATE;
+
+  item->print(out, QT_EXPLAIN);
+  thd->variables.option_bits= save_option_bits;
+}
 
 void Explain_table_access::tag_to_json(Json_writer *writer, enum explain_extra_tag tag)
 {
@@ -1409,15 +1453,6 @@ void add_json_keyset(Json_writer *writer, const char *elem_name,
     print_json_array(writer, elem_name, *keyset);
 }
 
-/*
-  @param fs_tracker   Normally NULL. When not NULL, it means that the join tab
-                      used filesort to pre-sort the data. Then, sorted data 
-                      was read and the rest of the join was executed.
-
-  @note
-  EXPLAIN command will check whether fs_tracker is present, but it can't use 
-  any value from fs_tracker (these are only valid for ANALYZE).
-*/
 
 void Explain_table_access::print_explain_json(Explain_query *query,
                                               Json_writer *writer,
@@ -1425,7 +1460,7 @@ void Explain_table_access::print_explain_json(Explain_query *query,
 {
   Json_writer_nesting_guard guard(writer);
   
-  if (using_filesort)
+  if (pre_join_sort)
   {
     /* filesort was invoked on this join tab before doing the join with the rest */
     writer->add_member("read_sorted_file").start_object();
@@ -1452,9 +1487,7 @@ void Explain_table_access::print_explain_json(Explain_query *query,
       }
     }
     writer->add_member("filesort").start_object();
-
-    if (is_analyze)
-      fs_tracker->print_json_members(writer);
+    pre_join_sort->print_json_members(writer, is_analyze);
   }
 
   if (bka_type.is_using_jbuf())
@@ -1532,11 +1565,11 @@ void Explain_table_access::print_explain_json(Explain_query *query,
   if (is_analyze)
   {
     writer->add_member("r_rows");
-    if (fs_tracker)
+    if (pre_join_sort)
     {
       /* Get r_rows value from filesort */
-      if (fs_tracker->get_r_loops())
-        writer->add_double(fs_tracker->get_avg_examined_rows());
+      if (pre_join_sort->tracker.get_r_loops())
+        writer->add_double(pre_join_sort->tracker.get_avg_examined_rows());
       else
         writer->add_null();
     }
@@ -1563,11 +1596,11 @@ void Explain_table_access::print_explain_json(Explain_query *query,
   if (is_analyze)
   {
     writer->add_member("r_filtered");
-    if (fs_tracker)
+    if (pre_join_sort)
     {
       /* Get r_filtered value from filesort */
-      if (fs_tracker->get_r_loops())
-        writer->add_double(fs_tracker->get_r_filtered());
+      if (pre_join_sort->tracker.get_r_loops())
+        writer->add_double(pre_join_sort->tracker.get_r_filtered());
       else
         writer->add_null();
     }
@@ -1645,7 +1678,7 @@ void Explain_table_access::print_explain_json(Explain_query *query,
     writer->end_object();
   }
 
-  if (using_filesort)
+  if (pre_join_sort)
   {
     writer->end_object(); // filesort
     writer->end_object(); // read_sorted_file

sql/sql_explain.h

@@ -281,9 +281,18 @@ class Explain_aggr_node : public Sql_alloc
 
 class Explain_aggr_filesort : public Explain_aggr_node 
 {
+  List<Item> sort_items;
 public:
   enum_explain_aggr_node_type get_type() { return AGGR_OP_FILESORT; }
-  Filesort_tracker *tracker;
+  Filesort_tracker tracker;
+
+  Explain_aggr_filesort(bool is_analyze) : tracker(is_analyze)
+  {
+    child= NULL;
+  }
+
+  void init(THD* thd, Filesort *filesort);
+  void print_json_members(Json_writer *writer, bool is_analyze);
 };
 
 class Explain_aggr_tmp_table : public Explain_aggr_node
@@ -663,8 +672,7 @@ class Explain_table_access : public Sql_alloc
     cache_cond(NULL),
     pushed_index_cond(NULL),
     sjm_nest(NULL),
-    using_filesort(false),
-    fs_tracker(NULL)
+    pre_join_sort(NULL)
   {}
   ~Explain_table_access() { delete sjm_nest; }
 
@@ -757,9 +765,13 @@ class Explain_table_access : public Sql_alloc
   Item *pushed_index_cond;
 
   Explain_basic_join *sjm_nest;
-   
-  bool using_filesort;
-  Filesort_tracker *fs_tracker;
+  
+  /*
+    This describes a possible filesort() call that is done before doing the
+    join operation.
+  */
+  Explain_aggr_filesort *pre_join_sort;
+
   /* ANALYZE members */
 
   /* Tracker for reading the table */

sql/sql_select.cc

@@ -23746,7 +23746,6 @@ void JOIN_TAB::save_explain_data(Explain_table_access *eta,
   explain_plan= eta;
   eta->key.clear();
   eta->quick_info= NULL;
-  eta->using_filesort= false;
 
   SQL_SELECT *tab_select;
   /* 
@@ -23758,9 +23757,8 @@ void JOIN_TAB::save_explain_data(Explain_table_access *eta,
 
   if (filesort)
   {
-    eta->using_filesort= true; // This fixes EXPLAIN
-    eta->fs_tracker= filesort->tracker= 
-      new Filesort_tracker(thd->lex->analyze_stmt);
+    eta->pre_join_sort= new Explain_aggr_filesort(thd->lex->analyze_stmt);
+    eta->pre_join_sort->init(thd, filesort);
   }
   
   tracker= &eta->tracker;
@@ -24201,9 +24199,9 @@ void save_agg_explain_data(JOIN *join, Explain_select *xpl_sel)
 
     if (join_tab->filesort)
     {
-      Explain_aggr_filesort *eaf = new Explain_aggr_filesort;
-      eaf->tracker= new Filesort_tracker(join->thd->lex->analyze_stmt);
-      join_tab->filesort->tracker= eaf->tracker;
+      bool is_analyze= join->thd->lex->analyze_stmt;
+      Explain_aggr_filesort *eaf = new Explain_aggr_filesort(is_analyze);
+      eaf->init(join->thd, join_tab->filesort);
 
       prev_node= node;
       node= eaf;