Help on analyzing performance problem.

I have a problem with a _very_ slow query. I'm not sure how to understand
the explain output. Can anyone tell from the following where I might look
for a bottleneck in the system?

Thank,
Travis

psql:test.sql:1: NOTICE: QUERY PLAN:

Nested Loop (cost=3.03..240.90 rows=4 width=124)
-> Nested Loop (cost=3.03..150.16 rows=7 width=92)
-> Merge Join (cost=3.03..3.63 rows=11 width=76)
-> Sort (cost=1.52..1.52 rows=17 width=32)
-> Seq Scan on oilday (cost=0.00..1.17 rows=17
width=32)
-> Sort (cost=1.52..1.52 rows=17 width=44)
-> Seq Scan on oilday (cost=0.00..1.17 rows=17
width=44)
-> Index Scan using well_pkey on well (cost=0.00..13.45 rows=1
width=16)
SubPlan
-> Materialize (cost=11.44..11.44 rows=82 width=12)
-> Nested Loop (cost=0.00..11.44 rows=82 width=12)
-> Seq Scan on usernames (cost=0.00..1.03
rows=1 width=4)
-> Index Scan using comp_index on well
(cost=0.00..10.21 rows=15 width=8)
-> Index Scan using well_pkey on well (cost=0.00..13.45 rows=1 width=32)
SubPlan
-> Materialize (cost=11.44..11.44 rows=82 width=12)
-> Nested Loop (cost=0.00..11.44 rows=82 width=12)
-> Seq Scan on usernames (cost=0.00..1.03 rows=1
width=4)
-> Index Scan using comp_index on well
(cost=0.00..10.21 rows=15 width=8)