Change Last_query_cost status variable from global to thread-local.

sql/mysql_priv.h:
  Changed last_query_cost status variable from global to thread-local.
sql/mysqld.cc:
  Changed last_query_cost status variable from global to thread-local.
sql/sql_class.cc:
  Changed last_query_cost status variable from global to thread-local.
sql/sql_class.h:
  Changed last_query_cost status variable from global to thread-local.
sql/sql_select.cc:
  Changed last_query_cost status variable from global to thread-local.
sql/sql_show.cc:
  All SHOW_DOUBLE-type status variables are now assumed to be thread-local (there is 
  only one currently, last_query_cost, and this cset makes it thread-local)

sql/mysql_priv.h

@@ -1048,7 +1048,6 @@ extern char language[FN_REFLEN], reg_ext[FN_EXTLEN];
 extern char glob_hostname[FN_REFLEN], mysql_home[FN_REFLEN];
 extern char pidfile_name[FN_REFLEN], system_time_zone[30], *opt_init_file;
 extern char log_error_file[FN_REFLEN], *opt_tc_log_file;
-extern double last_query_cost;
 extern double log_10[32];
 extern ulonglong log_10_int[20];
 extern ulonglong keybuff_size;

sql/mysqld.cc

@@ -345,7 +345,6 @@ ulong expire_logs_days = 0;
 ulong rpl_recovery_rank=0;
 ulong my_bind_addr;			/* the address we bind to */
 volatile ulong cached_thread_count= 0;
-double last_query_cost= -1; /* -1 denotes that no query was compiled yet */
 
 double log_10[32];			/* 10 potences */
 time_t start_time;
@@ -5714,7 +5713,7 @@ struct show_var_st status_vars[]= {
   {"Key_reads",                (char*) &dflt_key_cache_var.global_cache_read, SHOW_KEY_CACHE_LONG},
   {"Key_write_requests",       (char*) &dflt_key_cache_var.global_cache_w_requests, SHOW_KEY_CACHE_LONG},
   {"Key_writes",               (char*) &dflt_key_cache_var.global_cache_write, SHOW_KEY_CACHE_LONG},
-  {"Last_query_cost",          (char*) &last_query_cost,        SHOW_DOUBLE},
+  {"Last_query_cost",          (char*) offsetof(STATUS_VAR, last_query_cost), SHOW_DOUBLE},
   {"Max_used_connections",     (char*) &max_used_connections,  SHOW_LONG},
 #ifdef HAVE_NDBCLUSTER_DB
   {"Ndb_",                     (char*) &ndb_status_variables,   SHOW_VARS},

sql/sql_class.cc

@@ -444,6 +444,7 @@ void add_to_status(STATUS_VAR *to_var, STATUS_VAR *from_var)
 
   while (to != end)
     *(to++)+= *(from++);
+  /* it doesn't make sense to add last_query_cost values */
 }
 
 

sql/sql_class.h

@@ -635,6 +635,8 @@ typedef struct system_status_var
   ulong filesort_range_count;
   ulong filesort_rows;
   ulong filesort_scan_count;
+
+  double last_query_cost;
 } STATUS_VAR;
 
 /*

sql/sql_select.cc

@@ -3739,8 +3739,7 @@ choose_plan(JOIN *join, table_map join_tables)
     Don't update last_query_cost for 'show status' command
   */
   if (join->thd->lex->orig_sql_command != SQLCOM_SHOW_STATUS)
-    last_query_cost= join->best_read;
-
+    join->thd->status_var.last_query_cost= join->best_read;
   DBUG_VOID_RETURN;
 }
 

sql/sql_show.cc

@@ -1358,6 +1358,7 @@ static bool show_status_array(THD *thd, const char *wild,
         }
         case SHOW_DOUBLE:
         {
+          value= ((char *) status_var + (ulong) value);
           end= buff + sprintf(buff, "%f", *(double*) value);
           break;
         }