MDEV-3798: EXPLAIN UPDATE/DELETE

- if EXPLAIN DELETE prints "Deleting all rows", it should show the 
  expected number of rows in the rows column.

sql/sql_delete.cc

@@ -59,6 +59,7 @@ void Delete_plan::save_explain_data(Explain_query *query)
   {
     explain->deleting_all_rows= true;
     explain->select_type= "SIMPLE";
+    explain->rows= table_rows;
   }
   else
   {

sql/sql_explain.cc

@@ -778,7 +778,7 @@ int Explain_delete::print_explain(Explain_query *query,
     const char *msg= "Deleting all rows";
     int res= print_explain_message_line(output, explain_flags,
                                         1 /*select number*/,
-                                        select_type, msg);
+                                        select_type, &rows, msg);
     return res;
 
   }
@@ -797,11 +797,13 @@ int Explain_update::print_explain(Explain_query *query,
   if (impossible_where || no_partitions)
   {
     const char *msg= impossible_where ? 
-                     "Impossible where" : 
+                     "Impossible WHERE" : 
                      "No matching rows after partition pruning";
     int res= print_explain_message_line(output, explain_flags,
                                         1 /*select number*/,
-                                        select_type, msg);
+                                        select_type, 
+                                        NULL, /* rows */
+                                        msg);
     return res;
   }
 

sql/sql_select.cc

@@ -22222,6 +22222,7 @@ int print_explain_message_line(select_result_sink *result,
                                uint8 options,
                                uint select_number,
                                const char *select_type,
+                               ha_rows *rows,
                                const char *message)
 {
   const CHARSET_INFO *cs= system_charset_info;
@@ -22231,13 +22232,31 @@ int print_explain_message_line(select_result_sink *result,
   item_list.push_back(new Item_int((int32) select_number));
   item_list.push_back(new Item_string(select_type,
                                       strlen(select_type), cs));
-  for (uint i=0 ; i < 7; i++)
+  /* `table` */
   item_list.push_back(item_null);
+  
+  /* `partitions` */
   if (options & DESCRIBE_PARTITIONS)
     item_list.push_back(item_null);
+  
+  /* type, possible_keys, key, key_len, ref */
+  for (uint i=0 ; i < 5; i++)
+    item_list.push_back(item_null);
+
+  /* `rows` */
+  if (rows)
+  {
+    item_list.push_back(new Item_int(*rows, 
+                                     MY_INT64_NUM_DECIMAL_DIGITS));
+  }
+  else
+    item_list.push_back(item_null);
+
+  /* `filtered` */
   if (options & DESCRIBE_EXTENDED)
     item_list.push_back(item_null);
 
+  /* `Extra` */
   if (message)
     item_list.push_back(new Item_string(message,strlen(message),cs));
   else

sql/sql_select.h

@@ -1852,6 +1852,7 @@ int print_explain_message_line(select_result_sink *result,
                                uint8 options,
                                uint select_number,
                                const char *select_type,
+                               ha_rows *rows,
                                const char *message);
 void explain_append_mrr_info(QUICK_RANGE_SELECT *quick, String *res);
 int print_explain_row(select_result_sink *result,