MDEV-13049 Querying INFORMATION_SCHEMA becomes slow in MariaDB 10.1

Optimizations:
* avoid faster ext_table_discovery_simple() *only* when there are
  engines with discover_table_names() method *and* they look at
  file names. P_S implements discover_table_names(), but it's not
  a reason to use slower extension_based_table_discovery().
* don't pre-sort table names if ORDER BY or GROUP BY was specified
* starting from 10.3 only sort table names for SHOW commands

sql/handler.cc

@@ -405,7 +405,7 @@ static int ha_finish_errors(void)
 }
 
 static volatile int32 need_full_discover_for_existence= 0;
-static volatile int32 engines_with_discover_table_names= 0;
+static volatile int32 engines_with_discover_file_names= 0;
 static volatile int32 engines_with_discover= 0;
 
 static int full_discover_for_existence(handlerton *, const char *, const char *)
@@ -430,8 +430,8 @@ static void update_discovery_counters(handlerton *hton, int val)
   if (hton->discover_table_existence == full_discover_for_existence)
     my_atomic_add32(&need_full_discover_for_existence,  val);
 
-  if (hton->discover_table_names)
-    my_atomic_add32(&engines_with_discover_table_names, val);
+  if (hton->discover_table_names && hton->tablefile_extensions[0])
+    my_atomic_add32(&engines_with_discover_file_names, val);
 
   if (hton->discover_table)
     my_atomic_add32(&engines_with_discover, val);
@@ -5189,6 +5189,7 @@ void Discovered_table_list::remove_duplicates()
 {
   LEX_STRING **src= tables->front();
   LEX_STRING **dst= src;
+  sort();
   while (++dst <= tables->back())
   {
     LEX_STRING *s= *src, *d= *dst;
@@ -5256,10 +5257,12 @@ int ha_discover_table_names(THD *thd, LEX_STRING *db, MY_DIR *dirp,
   int error;
   DBUG_ENTER("ha_discover_table_names");
 
-  if (engines_with_discover_table_names == 0 && !reusable)
+  if (engines_with_discover_file_names == 0 && !reusable)
   {
-    error= ext_table_discovery_simple(dirp, result);
-    result->sort();
+    st_discover_names_args args= {db, NULL, result, 0};
+    error= ext_table_discovery_simple(dirp, result) ||
+           plugin_foreach(thd, discover_names,
+                            MYSQL_STORAGE_ENGINE_PLUGIN, &args);
   }
   else
   {
@@ -5272,8 +5275,6 @@ int ha_discover_table_names(THD *thd, LEX_STRING *db, MY_DIR *dirp,
     error= extension_based_table_discovery(dirp, reg_ext, result) ||
            plugin_foreach(thd, discover_names,
                             MYSQL_STORAGE_ENGINE_PLUGIN, &args);
-    result->sort();
-
     if (args.possible_duplicates > 0)
       result->remove_duplicates();
   }

sql/sql_show.cc

@@ -979,13 +979,20 @@ find_files(THD *thd, Dynamic_array<LEX_STRING*> *files, LEX_STRING *db,
       if (tl.add_file(file->name))
         goto err;
     }
-    tl.sort();
   }
   else
   {
     if (ha_discover_table_names(thd, db, dirp, &tl, false))
       goto err;
   }
+#if MYSQL_VERSION_ID < 100300
+  /* incomplete optimization, but a less drastic change in GA version */
+  if (!thd->lex->select_lex.order_list.elements &&
+      !thd->lex->select_lex.group_list.elements)
+#else
+  if (is_show_command(thd))
+#endif
+    tl.sort();
 
   DBUG_PRINT("info",("found: %zu files", files->elements()));
   my_dirend(dirp);