[PERFORM] adding 'limit' leads to very slow query

[Michael McFarland]

  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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