MDEV-3798: EXPLAIN UPDATE/DELETE

Update the SHOW EXPLAIN code to work with the 
new architecture (part#1):
Before, SHOW EXPLAIN operated on real query plan structures, 
which meant it had to check when SELECTs are created/deleted.
SELECTs would call apc_target->enable() when they got a query 
plan and disable() when their query plan was deleted.

Now, Explain data structure becomes available at once (and we
call apc_target->enable()) and then it stays until it is deleted
(when that happens, we call apc_target->disable()).

1 files changed, 6 insertions, 2 deletions

diff --git a/sql/sql_lex.cc b/sql/sql_lex.cc
index bd31200e749..09cb8f06afc 100644
--- a/sql/sql_lex.cc
+++ b/sql/sql_lex.cc
@@ -4276,11 +4276,15 @@ int st_select_lex_unit::save_union_explain(Explain_query *output)
   for (SELECT_LEX *sl= first; sl; sl= sl->next_select())
     eu->add_select(sl->select_number);
 
+  eu->fake_select_type= "UNION RESULT";
+  eu->using_filesort= test(global_parameters->order_list.first);
+
   // Save the UNION node
   output->add_node(eu);
 
-  eu->fake_select_type= "UNION RESULT";
-  eu->using_filesort= test(global_parameters->order_list.first);
+  if (eu->get_select_id() == 1)
+    output->query_plan_ready();
+
   return 0;
 }