MDEV-3798: EXPLAIN UPDATE/DELETE

- Run probes like MYSQL_INSERT_SELECT_START or 
  MYSQL_MULTI_DELETE_START for EXPLAIN, too.
  We should run them, because 1. EXPLAIN SELECT does it, and 
  2. MySQL also does it.

sql/sql_join_cache.cc

@@ -2553,16 +2553,16 @@ enum_nested_loop_state JOIN_CACHE::join_null_complements(bool skip_last)
 
 
 /*
-  Add a comment on the join algorithm employed by the join cache 
+  Save data on the join algorithm employed by the join cache 
 
   SYNOPSIS
-    print_explain_comment()
+    save_explain_data()
       str  string to add the comment on the employed join algorithm to
 
   DESCRIPTION
-    This function adds info on the type of the used join buffer (flat or
+    This function puts info about the type of the used join buffer (flat or
     incremental) and on the type of the the employed join algorithm (BNL,
-    BNLH, BKA or BKAH) to the the end of the sring str.
+    BNLH, BKA or BKAH) to the data structure
 
   RETURN VALUE
     none

sql/sql_parse.cc

@@ -3205,11 +3205,8 @@ case SQLCOM_PREPARE:
     unit->set_limit(select_lex);
 
     if (!(res= open_and_lock_tables(thd, all_tables, TRUE, 0)))
-    {
-      if (!explain)
     {
       MYSQL_INSERT_SELECT_START(thd->query());
-      }
       /*
         Only the INSERT table should be merged. Other will be handled by
         select.
@@ -3258,10 +3255,7 @@ case SQLCOM_PREPARE:
       }
 
       /* revert changes for SP */
-      if (!explain)
-      {
       MYSQL_INSERT_SELECT_DONE(res, (ulong) thd->get_row_count_func());
-      }
       select_lex->table_list.first= first_table;
     }
     /*
@@ -3316,17 +3310,10 @@ case SQLCOM_PREPARE:
     if ((res= open_and_lock_tables(thd, all_tables, TRUE, 0)))
       break;
 
-    if (!explain)
-    {
     MYSQL_MULTI_DELETE_START(thd->query());
-    }
-
     if ((res= mysql_multi_delete_prepare(thd)))
-    {
-      if (!explain)
     {
       MYSQL_MULTI_DELETE_DONE(1, 0);
-      }
       goto error;
     }
 
@@ -3348,15 +3335,12 @@ case SQLCOM_PREPARE:
                           result, unit, select_lex);
         res|= thd->is_error();
 
-        if (!explain)
-        {
         MYSQL_MULTI_DELETE_DONE(res, del_result->num_deleted());
         if (res)
           result->abort_result_set(); /* for both DELETE and EXPLAIN DELETE */
-        }
         else
         {
-          if (!res)
+          if (explain)
           {
             select_result *result= new select_send();
             LEX *lex= thd->lex;
@@ -3365,8 +3349,6 @@ case SQLCOM_PREPARE:
                 result->send_eof())
               res= 1;
           }
-          else
-            result->abort_result_set(); /* for both DELETE and EXPLAIN DELETE */
         }
         delete result;
       }
@@ -3374,11 +3356,8 @@ case SQLCOM_PREPARE:
     else
     {
       res= TRUE;                                // Error
-      if (!explain)
-      {
       MYSQL_MULTI_DELETE_DONE(1, 0);
     }
-    }
     break;
   }
   case SQLCOM_DROP_TABLE: