MDEV-17251 SHOW STATUS unnecessary calls calc_sum_of_all_status

1. only call calc_sum_of_all_status() if a global
   SHOW_xxx_STATUS variable is to be returned
2. only lock LOCK_status when copying global_status_var,
   but not when iterating all threads

4 files changed, 16 insertions, 11 deletions

diff --git a/sql/sql_parse.cc b/sql/sql_parse.cc
index 54937116383..131ba4a86c5 100644
--- a/sql/sql_parse.cc
+++ b/sql/sql_parse.cc
@@ -2179,6 +2179,7 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
       break;
     general_log_print(thd, command, NullS);
     status_var_increment(thd->status_var.com_stat[SQLCOM_SHOW_STATUS]);
+    *current_global_status_var= global_status_var;
     calc_sum_of_all_status(current_global_status_var);
     if (!(uptime= (ulong) (thd->start_time - server_start_time)))
       queries_per_second1000= 0;
diff --git a/sql/sql_plugin.h b/sql/sql_plugin.h
index 64194b5a1b5..13f7296e0cd 100644
--- a/sql/sql_plugin.h
+++ b/sql/sql_plugin.h
@@ -22,9 +22,10 @@
   that is defined in plugin.h
 */
 #define SHOW_always_last SHOW_KEY_CACHE_LONG, \
-            SHOW_LONG_STATUS, SHOW_DOUBLE_STATUS, \
             SHOW_HAVE, SHOW_MY_BOOL, SHOW_HA_ROWS, SHOW_SYS, \
-            SHOW_LONG_NOFLUSH, SHOW_LONGLONG_STATUS, SHOW_LEX_STRING
+            SHOW_LONG_NOFLUSH, SHOW_LEX_STRING, \
+      /* SHOW_*_STATUS must be at the end, SHOW_LONG_STATUS being first */ \
+            SHOW_LONG_STATUS, SHOW_DOUBLE_STATUS, SHOW_LONGLONG_STATUS
 #include <my_global.h>
 #undef SHOW_always_last
 
diff --git a/sql/sql_show.cc b/sql/sql_show.cc
index 79897429a4f..4294157cce3 100644
--- a/sql/sql_show.cc
+++ b/sql/sql_show.cc
@@ -3580,6 +3580,16 @@ static bool show_status_array(THD *thd, const char *wild,
 
         if (show_type == SHOW_SYS)
           mysql_mutex_lock(&LOCK_global_system_variables);
+        else if (show_type >= SHOW_LONG_STATUS && scope == OPT_GLOBAL &&
+                 !status_var->local_memory_used)
+        {
+          mysql_mutex_lock(&LOCK_status);
+          *status_var= global_status_var;
+          mysql_mutex_unlock(&LOCK_status);
+          calc_sum_of_all_status(status_var);
+          DBUG_ASSERT(status_var->local_memory_used);
+        }
+
         pos= get_one_variable(thd, var, scope, show_type, status_var,
                               &charset, buff, &length);
 
@@ -3620,8 +3630,6 @@ uint calc_sum_of_all_status(STATUS_VAR *to)
   I_List_iterator<THD> it(threads);
   THD *tmp;
 
-  /* Get global values as base */
-  *to= global_status_var;
   to->local_memory_used= 0;
 
   /* Add to this status from existing threads */
@@ -7586,13 +7594,7 @@ int fill_status(THD *thd, TABLE_LIST *tables, COND *cond)
   if (partial_cond)
     partial_cond->val_int();
 
-  if (scope == OPT_GLOBAL)
-  {
-    /* We only hold LOCK_status for summary status vars */
-    mysql_mutex_lock(&LOCK_status);
-    calc_sum_of_all_status(&tmp);
-    mysql_mutex_unlock(&LOCK_status);
-  }
+  tmp.local_memory_used= 0; // meaning tmp was not populated yet
 
   mysql_mutex_lock(&LOCK_show_status);
   res= show_status_array(thd, wild,
diff --git a/sql/sql_test.cc b/sql/sql_test.cc
index 4e68ec2ec2e..c0e62227665 100644
--- a/sql/sql_test.cc
+++ b/sql/sql_test.cc
@@ -564,6 +564,7 @@ void mysql_print_status()
   STATUS_VAR tmp;
   uint count;
 
+  tmp= global_status_var;
   count= calc_sum_of_all_status(&tmp);
   printf("\nStatus information:\n\n");
   (void) my_getwd(current_dir, sizeof(current_dir),MYF(0));