do not use bulk insert tree code if ``myisam_bulk_insert_tree_size'' is too small

myisam/mi_write.c

@@ -821,7 +821,8 @@ int _mi_init_bulk_insert(MI_INFO *info)
     }
   }
 
-  if (num_keys==0 || num_keys>myisam_bulk_insert_tree_size)
+  if (num_keys==0 ||
+      num_keys * MI_MIN_SIZE_BULK_INSERT_TREE > myisam_bulk_insert_tree_size)
     return 0;
 
   info->bulk_insert=(TREE *)

myisam/myisamdef.h

@@ -373,6 +373,8 @@ struct st_myisam_info {
 #define MI_MAX_KEYPTR_SIZE	5        /* For calculating block lengths */
 #define MI_MIN_KEYBLOCK_LENGTH	50         /* When to split delete blocks */
 
+#define MI_MIN_SIZE_BULK_INSERT_TREE 16384             /* this is per key */
+
 /* The UNIQUE check is done with a hashed long key */
 
 #define MI_UNIQUE_HASH_TYPE	HA_KEYTYPE_ULONG_INT