questions on interpreting the output of explain

I'm trying to optimize a query and don't yet understand PostGreSQL's
query optimization... I'm not sure how to read it in terms of best
outcome. On a query that looks like:

explain select U.oid from users U, selections S where
  U.acctname = S.acctname and
  U.birthdate <= '1-1-1963' and
  U.birthdate >= '1-1-1933' and
  38 >= S.field1_int  and
  38 <= S.field2_int and
  U.tol_a < 99.0 and
  U.tol_b < 99.0

(indexes on: U.birthdate, U.acctname, S.acctname, U.tol_a, U.tol_b)

I get:

Nested Loop  (cost=9.38 size=1 width=28)
  ->  Index Scan on u  (cost=3.26 size=3 width=16)
  ->  Index Scan on s  (cost=2.04 size=25 width=12)

...but if I shorten the query slightly:

explain select U.oid from users U, selections S where
  U.acctname = S.acctname and
  U.birthdate <= '1-1-1963' and
  U.birthdate >= '1-1-1933' and
  38 >= S.field1_int  and
  38 <= S.field2_int

...I get:

Hash Join  (cost=14.67 size=3 width=28)
  ->  Index Scan on u  (cost=3.26 size=26 width=16)
  ->  Hash  (cost=0.00 size=0 width=0)
        ->  Index Scan on s  (cost=7.73 size=25 width=12)

Can someone explain the details here to me? I guess specific questions
are:
- Does hash join mean my acctname indexes are hashed, not btree'd?
- What does index scan mean?
- Nested loop?
- Does the cost roll-up, meaning the top line is total cost, and the
  rest is a breakdown, or do I add up all the cost numbers?
- Can I compare costs between two queries?

thanks!
Michael

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