BUG#20483278 LAST_QUERY_COST SHOWS DIFFERENT VALUES FOR SHOW_COMPATIBLITY

FOR ON VS OFF
BUG#21788549 LAST_QUERY_PARTIAL_PLANS VALUES NOT SAME WITH
SHOW_COMPATIBILITY_56

Before this fix,

With SHOW_COMPATIBILITY_56=ON,

The statement
  SHOW STATUS like 'Last_query_cost';
was executed with dedicated code in the information schema
(sql/sql_show.cc), which is -- not -- implemented as a storage engine.
As a result, this statement did not execute code in the optimizer,
which did not overwrite the session status variable 'Last_query_cost'
for the current select statement.

As a result, the value of 'Last_query_cost' reported was
for the previous SELECT statement executed in the session.

With SHOW_COMPATIBILITY_56=OFF,
  SHOW STATUS like 'Last_query_cost';
is executed as a SELECT ... FROM performance_schema.session_status.
This table is implemented with a storage engine,
and the optimizer code is used.
By the time the value is returned, the value of 'Last_query_cost'
reported is the value just overwritten by the optimizer for the current
statement, which is not the desired result.

The fix is to separate clearly in the session attributes:
- the value of 'Last_query_cost' for the current query,
- the value of 'Last_query_cost' for the previous query,
and only save the former in the later when query execution is complete.

The same issue exists for Last_query_partial_plan,
which is fixed the same way.

Author: marcalff

mysql-test/suite/perfschema/r/show_misc.result

@@ -0,0 +1,39 @@
+select @@global.show_compatibility_56 into @show_compatibility_56_save;
+================================================================================
+SHOW_COMPATIBILITY_56 = ON
+================================================================================
+set global show_compatibility_56 = on;
+select 1;
+1
+1
+SHOW STATUS LIKE 'Last_query_partial_plans';
+Variable_name	Value
+Last_query_partial_plans	0
+select 2;
+2
+2
+SHOW STATUS LIKE 'Last_query_cost';
+Variable_name	Value
+Last_query_cost	0.000000
+
+================================================================================
+SHOW_COMPATIBILITY_56 = OFF
+================================================================================
+set global show_compatibility_56 = off;
+select 1;
+1
+1
+SHOW STATUS LIKE 'Last_query_partial_plans';
+Variable_name	Value
+Last_query_partial_plans	0
+select 2;
+2
+2
+SHOW STATUS LIKE 'Last_query_cost';
+Variable_name	Value
+Last_query_cost	0.000000
+
+================================================================================
+CLEAN UP
+================================================================================
+set @@global.show_compatibility_56 = @show_compatibility_56_save;

mysql-test/suite/perfschema/t/show_misc.test

@@ -0,0 +1,34 @@
+--source include/have_perfschema.inc
+--source include/not_embedded.inc
+--source include/no_protocol.inc
+
+select @@global.show_compatibility_56 into @show_compatibility_56_save;
+
+--echo ================================================================================
+--echo SHOW_COMPATIBILITY_56 = ON
+--echo ================================================================================
+set global show_compatibility_56 = on;
+
+select 1;
+SHOW STATUS LIKE 'Last_query_partial_plans';
+select 2;
+SHOW STATUS LIKE 'Last_query_cost';
+
+
+--echo
+--echo ================================================================================
+--echo SHOW_COMPATIBILITY_56 = OFF
+--echo ================================================================================
+set global show_compatibility_56 = off;
+
+select 1;
+SHOW STATUS LIKE 'Last_query_partial_plans';
+select 2;
+SHOW STATUS LIKE 'Last_query_cost';
+
+--echo
+--echo ================================================================================
+--echo CLEAN UP
+--echo ================================================================================
+set @@global.show_compatibility_56 = @show_compatibility_56_save;
+

sql/sql_cache.cc

@@ -1906,7 +1906,8 @@ def_week_frmt: %lu, in_trans: %d, autocommit: %d",
 #endif /*!EMBEDDED_LIBRARY*/
 
   thd->limit_found_rows = query->found_rows();
-  thd->status_var.last_query_cost= 0.0;
+  thd->clear_current_query_costs();
+  thd->save_current_query_costs();
 
   {
     Opt_trace_start ots(thd, NULL, SQLCOM_SELECT, NULL,

sql/sql_class.h

@@ -1550,6 +1550,43 @@ class THD :public MDL_context_owner,
   struct  system_status_var *initial_status_var; /* used by show status */
   // has status_var already been added to global_status_var?
   bool status_var_aggregated;
+
+  /**
+    Current query cost.
+    @sa system_status_var::last_query_cost
+  */
+  double m_current_query_cost;
+  /**
+    Current query partial plans.
+    @sa system_status_var::last_query_partial_plans
+  */
+  ulonglong m_current_query_partial_plans;
+
+  /**
+    Clear the query costs attributes for the current query.
+  */
+  void clear_current_query_costs()
+  {
+    m_current_query_cost= 0.0;
+    m_current_query_partial_plans= 0;
+  }
+
+  /**
+    Save the current query costs attributes in
+    the thread session status.
+    Use this method only after the query execution is completed,
+    so that
+      @code SHOW SESSION STATUS like 'last_query_%' @endcode
+      @code SELECT * from performance_schema.session_status
+      WHERE VARIABLE_NAME like 'last_query_%' @endcode
+    actually reports the previous query, not itself.
+  */
+  void save_current_query_costs()
+  {
+    status_var.last_query_cost= m_current_query_cost;
+    status_var.last_query_partial_plans= m_current_query_partial_plans;
+  }
+
   THR_LOCK_INFO lock_info;              // Locking info of this thread
   /**
     Protects THD data accessed from other threads.

sql/sql_optimizer.cc

@@ -690,13 +690,13 @@ JOIN::optimize()
       DBUG_RETURN(1);
   }
 
-  // Update last_query_cost to reflect actual need of filesort.
+  // Update m_current_query_cost to reflect actual need of filesort.
   if (sort_cost > 0.0 && !explain_flags.any(ESP_USING_FILESORT))
   {
     best_read-= sort_cost;
     sort_cost= 0.0;
     if (thd->lex->is_single_level_stmt())
-      thd->status_var.last_query_cost= best_read;
+      thd->m_current_query_cost= best_read;
   }
 
   count_field_types(select_lex, &tmp_table_param, all_fields, false, false);
@@ -5030,12 +5030,12 @@ bool JOIN::make_join_plan()
 
   /*
     Store the cost of this query into a user variable
-    Don't update last_query_cost for statements that are not "flat joins" :
+    Don't update m_current_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 (thd->lex->is_single_level_stmt())
-    thd->status_var.last_query_cost= best_read;
+    thd->m_current_query_cost= best_read;
 
   // Generate an execution plan from the found optimal join order.
   if (get_best_combination())

sql/sql_parse.cc

@@ -2589,14 +2589,15 @@ mysql_execute_command(THD *thd, bool first_level)
     DBUG_EXECUTE_IF("use_attachable_trx",
                     thd->begin_attachable_transaction(););
 
-    thd->status_var.last_query_cost= 0.0;
-    thd->status_var.last_query_partial_plans= 0;
+    thd->clear_current_query_costs();
 
     res= select_precheck(thd, lex, all_tables, first_table);
 
     if (!res)
       res= execute_sqlcom_select(thd, all_tables);
 
+    thd->save_current_query_costs();
+
     DBUG_EXECUTE_IF("use_attachable_trx",
                     thd->end_attachable_transaction(););
 

sql/sql_planner.cc

@@ -914,7 +914,7 @@ void Optimize_table_order::best_access_path(JOIN_TAB *tab,
 
   float filter_effect= 1.0;
 
-  thd->status_var.last_query_partial_plans++;
+  thd->m_current_query_partial_plans++;
 
   /*
     Cannot use join buffering if either