Bug #30377: EXPLAIN loses last_query_cost when used with UNION

Currently the Last_query_cost session status variable shows
only the cost of a single flat subselect. For complex queries
(with subselects or unions etc) Last_query_cost is not valid
as it was showing the cost for the last optimized subselect.
Fixed by reseting to zero Last_query_cost when the complete
cost of the query cannot be determined.
Last_query_cost will be non-zero only for single flat queries.


mysql-test/r/status.result:
  Bug #30377: test case
mysql-test/t/status.test:
  Bug #30377: test case
sql/sql_lex.h:
  Bug #30377: helper function
sql/sql_select.cc:
  Bug #30377: don't assign cost if not on single level statement

2 files changed, 28 insertions, 2 deletions

diff --git a/sql/sql_lex.h b/sql/sql_lex.h
index 94015a9fe07..ce56be79744 100644
--- a/sql/sql_lex.h
+++ b/sql/sql_lex.h
@@ -1257,6 +1257,28 @@ typedef struct st_lex : public Query_tables_list
 
   void reset_n_backup_query_tables_list(Query_tables_list *backup);
   void restore_backup_query_tables_list(Query_tables_list *backup);
+
+  /**
+    @brief check if the statement is a single-level join
+    @return result of the check
+      @retval TRUE  The statement doesn't contain subqueries, unions and 
+                    stored procedure calls.
+      @retval FALSE There are subqueries, UNIONs or stored procedure calls.
+  */
+  bool is_single_level_stmt() 
+  { 
+    /* 
+      This check exploits the fact that the last added to all_select_list is
+      on its top. So select_lex (as the first added) will be at the tail 
+      of the list.
+    */ 
+    if (&select_lex == all_selects_list && !sroutines.records)
+    {
+      DBUG_ASSERT(!all_selects_list->next_select_in_list());
+      return TRUE;
+    }
+    return FALSE;
+  }
 } LEX;
 
 struct st_lex_local: public st_lex
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index b7846a7433d..7e9cb4cfbec 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -4369,9 +4369,13 @@ choose_plan(JOIN *join, table_map join_tables)
 
   /* 
     Store the cost of this query into a user variable
-    Don't update last_query_cost for 'show status' command
+    Don't update last_query_cost for 'show status' command.
+    Don't update last_query_cost for statements that are not "flat joins" :
+    i.e. they have subqueries, unions or call stored procedures.
+    TODO: calculate a correct cost for a query with subqueries and UNIONs.
   */
-  if (join->thd->lex->orig_sql_command != SQLCOM_SHOW_STATUS)
+  if (join->thd->lex->orig_sql_command != SQLCOM_SHOW_STATUS &&
+      join->thd->lex->is_single_level_stmt())
     join->thd->status_var.last_query_cost= join->best_read;
   DBUG_RETURN(FALSE);
 }