MDEV-17096 Pushdown of simple derived tables to storage engines

Added plugin system variable federated_pushdown.

mysql-test/suite/federated/federatedx_create_handlers.test

 --source have_federatedx.inc
 --source include/federated.inc
 
+set global federated_pushdown=1;
+
 connection slave;
 
 DROP TABLE IF EXISTS federated.t1;
@@ -152,3 +154,5 @@ DROP TABLE federated.t1, federated.t2;
 connection default;
 
 source include/federated_cleanup.inc;
+
+set global federated_pushdown=0;

storage/federatedx/federatedx_pushdown.cc

@@ -19,6 +19,9 @@
 static derived_handler*
 create_federatedx_derived_handler(THD* thd, TABLE_LIST *derived)
 {
+  if (!use_pushdown)
+    return 0;
+
   ha_federatedx_derived_handler* handler = NULL;
   handlerton *ht= 0;
 
@@ -144,6 +147,9 @@ void ha_federatedx_derived_handler::print_error(int, unsigned long)
 static select_handler*
 create_federatedx_select_handler(THD* thd, SELECT_LEX *sel)
 {
+  if (!use_pushdown)
+    return 0;
+
   ha_federatedx_select_handler* handler = NULL;
   handlerton *ht= 0;
 

storage/federatedx/ha_federatedx.cc

@@ -3677,11 +3677,17 @@ int ha_federatedx::discover_assisted(handlerton *hton, THD* thd,
   return error;
 }
 
-#include "federatedx_pushdown.cc"
 
 struct st_mysql_storage_engine federatedx_storage_engine=
 { MYSQL_HANDLERTON_INTERFACE_VERSION };
 
+my_bool use_pushdown;
+static MYSQL_SYSVAR_BOOL(pushdown, use_pushdown, 0,
+  "Use query fragments pushdown capabilities", NULL, NULL, FALSE);
+static struct st_mysql_sys_var* sysvars[]= { MYSQL_SYSVAR(pushdown) };
+
+#include "federatedx_pushdown.cc"
+
 maria_declare_plugin(federatedx)
 {
   MYSQL_STORAGE_ENGINE_PLUGIN,
@@ -3694,7 +3700,7 @@ maria_declare_plugin(federatedx)
   federatedx_done, /* Plugin Deinit */
   0x0201 /* 2.1 */,
   NULL,                       /* status variables                */
-  NULL,                       /* system variables                */
+  sysvars,                    /* system variables                */
   "2.1",                      /* string version */
   MariaDB_PLUGIN_MATURITY_STABLE /* maturity */
 }