2 queries really fast, combined really slow

I’ve a table like so:

fro int(11),
nid int(11)

Primary key is fro, nid.
I have an index on fro and an index on nid
Storage engine is innodb; I’ve 3GB dedicated to innodb_buffer_pool_size.

These two queries are incredibly fast:

select nid from user_network where fro=506017534;

2 rows in set (0.00 sec)

select * from user_network where nid in (50431654, 67108872);

14590 rows in set (0.05 sec)

This query is unjustifiably slow.

select * from user_network where nid in (select nid from user_network where fro=506017534);
14590 rows in set (8.86 sec)

What’s going on? I’m beginning to think something is wrong with my disk…

Mysql does not support this. ver 5
So using 2 queries is the right choise.

Post the output from using EXPLAIN on your query. e.g “EXPLAIN SELECT …”
Without that output nobody knows how the DBMS is performing your query.