adding 'limit' leads to very slow query

I'm trying to understand why a particular query is slow, and it seems
like the optimizer is choosing a strange plan. See this summary:

* I have a large table, with an index on the primary key 'id' and on a
field 'foo'.
> select count(*) from foo;
1,000,000
> select count(*) from foo where bar = 41;
7

* This query happens very quickly.
> explain select * from foo where barId = 412 order by id desc;
Sort ()
Sort key= id
-> Index scan using bar_index on foo ()
Index cond: barId = 412

But this query takes forever

> explain select * from foo where barId = 412 order by id desc limit 25;
Limit ()
-> Index scan backward using primarykey_index
Filter: barID = 412

Could anyone shed some light on what might be happening here?

- Michael

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