Comment and test changes per review request by Timour. All tests pass on production with this code.

mysql-test/r/federated.result:
  added explain results to join test results per Timour's request.
mysql-test/t/federated.test:
  Added explain to test per Timour's request
sql/ha_federated.cc:
  better comment to explain why we need table->status set to 0

mysql-test/r/federated.result

@@ -906,6 +906,13 @@ INSERT INTO federated.t1 (name, country_id, other) VALUES ('Lenz', 2, 22222);
 INSERT INTO federated.t1 (name, country_id, other) VALUES ('Marizio', 3, 33333);
 INSERT INTO federated.t1 (name, country_id, other) VALUES ('Monty', 4, 33333);
 INSERT INTO federated.t1 (name, country_id, other) VALUES ('Sanja', 5, 33333);
+EXPLAIN SELECT federated.t1.name AS name, federated.t1.country_id AS country_id,
+federated.t1.other AS other, federated.countries.country AS country 
+FROM federated.t1, federated.countries WHERE
+federated.t1.country_id = federated.countries.id;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	countries	ALL	PRIMARY	NULL	NULL	NULL	5	
+1	SIMPLE	t1	ref	country_id	country_id	4	federated.countries.id	120	
 SELECT federated.t1.name AS name, federated.t1.country_id AS country_id,
 federated.t1.other AS other, federated.countries.country AS country 
 FROM federated.t1, federated.countries WHERE
@@ -916,6 +923,13 @@ Lenz	2	22222	Germany
 Marizio	3	33333	Italy
 Monty	4	33333	Finland
 Sanja	5	33333	Ukraine
+EXPLAIN SELECT federated.t1.name AS name, federated.t1.country_id AS country_id,
+federated.t1.other AS other, federated.countries.country AS country
+FROM federated.t1 INNER JOIN federated.countries ON
+federated.t1.country_id = federated.countries.id;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	countries	ALL	PRIMARY	NULL	NULL	NULL	5	
+1	SIMPLE	t1	ref	country_id	country_id	4	federated.countries.id	120	
 SELECT federated.t1.name AS name, federated.t1.country_id AS country_id,
 federated.t1.other AS other, federated.countries.country AS country
 FROM federated.t1 INNER JOIN federated.countries ON
@@ -926,6 +940,14 @@ Lenz	2	22222	Germany
 Marizio	3	33333	Italy
 Monty	4	33333	Finland
 Sanja	5	33333	Ukraine
+EXPLAIN SELECT federated.t1.name AS name, federated.t1.country_id AS country_id,
+federated.t1.other AS other, federated.countries.country AS country
+FROM federated.t1 INNER JOIN federated.countries ON
+federated.t1.country_id = federated.countries.id
+WHERE federated.t1.name = 'Monty';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	countries	ALL	PRIMARY	NULL	NULL	NULL	5	
+1	SIMPLE	t1	ref	country_id	country_id	4	federated.countries.id	120	Using where
 SELECT federated.t1.name AS name, federated.t1.country_id AS country_id,
 federated.t1.other AS other, federated.countries.country AS country
 FROM federated.t1 INNER JOIN federated.countries ON
@@ -933,6 +955,13 @@ federated.t1.country_id = federated.countries.id
 WHERE federated.t1.name = 'Monty';
 name	country_id	other	country
 Monty	4	33333	Finland
+EXPLAIN SELECT federated.t1.*, federated.countries.country 
+FROM federated.t1 LEFT JOIN federated.countries 
+ON federated.t1.country_id = federated.countries.id
+ORDER BY federated.countries.id;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	10000	Using temporary; Using filesort
+1	SIMPLE	countries	eq_ref	PRIMARY	PRIMARY	4	federated.t1.country_id	1	
 SELECT federated.t1.*, federated.countries.country 
 FROM federated.t1 LEFT JOIN federated.countries 
 ON federated.t1.country_id = federated.countries.id
@@ -943,6 +972,13 @@ id	country_id	name	other	country
 3	3	Marizio	33333	Italy
 4	4	Monty	33333	Finland
 5	5	Sanja	33333	Ukraine
+EXPLAIN SELECT federated.t1.*, federated.countries.country 
+FROM federated.t1 LEFT JOIN federated.countries 
+ON federated.t1.country_id = federated.countries.id
+ORDER BY federated.countries.country;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	10000	Using temporary; Using filesort
+1	SIMPLE	countries	eq_ref	PRIMARY	PRIMARY	4	federated.t1.country_id	1	
 SELECT federated.t1.*, federated.countries.country 
 FROM federated.t1 LEFT JOIN federated.countries 
 ON federated.t1.country_id = federated.countries.id
@@ -953,6 +989,13 @@ id	country_id	name	other	country
 1	1	Kumar	11111	India
 3	3	Marizio	33333	Italy
 5	5	Sanja	33333	Ukraine
+EXPLAIN SELECT federated.t1.*, federated.countries.country 
+FROM federated.t1 RIGHT JOIN federated.countries 
+ON federated.t1.country_id = federated.countries.id 
+ORDER BY federated.t1.country_id;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	countries	ALL	NULL	NULL	NULL	NULL	5	Using temporary; Using filesort
+1	SIMPLE	t1	ref	country_id	country_id	4	federated.countries.id	120	
 SELECT federated.t1.*, federated.countries.country 
 FROM federated.t1 RIGHT JOIN federated.countries 
 ON federated.t1.country_id = federated.countries.id 

mysql-test/t/federated.test

@@ -862,16 +862,32 @@ INSERT INTO federated.t1 (name, country_id, other) VALUES ('Monty', 4, 33333);
 INSERT INTO federated.t1 (name, country_id, other) VALUES ('Sanja', 5, 33333);
 
 #inner join
+EXPLAIN SELECT federated.t1.name AS name, federated.t1.country_id AS country_id,
+federated.t1.other AS other, federated.countries.country AS country 
+FROM federated.t1, federated.countries WHERE
+federated.t1.country_id = federated.countries.id;
+
 SELECT federated.t1.name AS name, federated.t1.country_id AS country_id,
 federated.t1.other AS other, federated.countries.country AS country 
 FROM federated.t1, federated.countries WHERE
 federated.t1.country_id = federated.countries.id;
 
+EXPLAIN SELECT federated.t1.name AS name, federated.t1.country_id AS country_id,
+federated.t1.other AS other, federated.countries.country AS country
+FROM federated.t1 INNER JOIN federated.countries ON
+federated.t1.country_id = federated.countries.id;
+
 SELECT federated.t1.name AS name, federated.t1.country_id AS country_id,
 federated.t1.other AS other, federated.countries.country AS country
 FROM federated.t1 INNER JOIN federated.countries ON
 federated.t1.country_id = federated.countries.id;
 
+EXPLAIN SELECT federated.t1.name AS name, federated.t1.country_id AS country_id,
+federated.t1.other AS other, federated.countries.country AS country
+FROM federated.t1 INNER JOIN federated.countries ON
+federated.t1.country_id = federated.countries.id
+WHERE federated.t1.name = 'Monty';
+
 SELECT federated.t1.name AS name, federated.t1.country_id AS country_id,
 federated.t1.other AS other, federated.countries.country AS country
 FROM federated.t1 INNER JOIN federated.countries ON
@@ -879,17 +895,32 @@ federated.t1.country_id = federated.countries.id
 WHERE federated.t1.name = 'Monty';
 
 #left join
+EXPLAIN SELECT federated.t1.*, federated.countries.country 
+FROM federated.t1 LEFT JOIN federated.countries 
+ON federated.t1.country_id = federated.countries.id
+ORDER BY federated.countries.id;
+
 SELECT federated.t1.*, federated.countries.country 
 FROM federated.t1 LEFT JOIN federated.countries 
 ON federated.t1.country_id = federated.countries.id
 ORDER BY federated.countries.id;
 
+EXPLAIN SELECT federated.t1.*, federated.countries.country 
+FROM federated.t1 LEFT JOIN federated.countries 
+ON federated.t1.country_id = federated.countries.id
+ORDER BY federated.countries.country;
+
 SELECT federated.t1.*, federated.countries.country 
 FROM federated.t1 LEFT JOIN federated.countries 
 ON federated.t1.country_id = federated.countries.id
 ORDER BY federated.countries.country;
 
 #right join
+EXPLAIN SELECT federated.t1.*, federated.countries.country 
+FROM federated.t1 RIGHT JOIN federated.countries 
+ON federated.t1.country_id = federated.countries.id 
+ORDER BY federated.t1.country_id;
+
 SELECT federated.t1.*, federated.countries.country 
 FROM federated.t1 RIGHT JOIN federated.countries 
 ON federated.t1.country_id = federated.countries.id 

sql/ha_federated.cc

@@ -1526,7 +1526,11 @@ int ha_federated::index_read_idx(byte *buf, uint index, const byte *key,
     table->status= STATUS_NOT_FOUND;
     DBUG_RETURN(mysql_errno(mysql));
   }
-  /* very important - joins will not work without this! */
+  /* 
+     This basically says that the record in table->record[0] is legal, and that it is
+     ok to use this record, for whatever reason, such as with a join (without it, joins 
+     will not work)
+  */
   table->status=0;
 
   DBUG_RETURN(rnd_next(buf));