what does "index map: N" mean?

Sometimes in EXPLAIN output, I see this in the Extra field:

range checked for each record (index map: N)

Here's an example:

drop table t1;
create table t1 (x int, y int, index(x), index(y)); insert into t1 (x) values (1),(2),(3),(4),(5),(6),(7),(8),(9); update t1 set y=x;
explain select * from t1, t1 t2 where t1.y = 2 and t2.x between 7 and t1.y+0;

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	ref	y	y	5	const	1	Using where
1	SIMPLE	t2	range	x	x	5	NULL	4	Range checked for each record (index map: 

I know what the rest of it means, but what does 0x1 mean?

--

Baron Schwartz
Xaprb LLC
http://www.xaprb.com/

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