Performance problem with 50,000,000 rows

I'm new to PG but this just seems wrong. Can someone take a look:

.-----------. .-----------.
| bk_inv | | bk_title |
|-----------| |-----------|
| isbn |<--->| isbn |
| store | | vendor |
| qty | | |
| week | `-----------'
| | 2,000,000 recs
`-----------'
50,000,000 recs

Actual record numbers:
bk_inv : 46,790,877
bk_title: 2,311,710

VENDOR REPORT

A list of Inventory items, for any one given vendor (e.q. 01672708)

select i.isbn,
t.vendor,
i.store,
i.qty
from bk_inv i,
bk_title t
where i.isbn = t.isbn
and t.vendor = '01672708' ;

This query should be instantaneous. Granted that's 50 million rows, but
I have created an index on the isbn column for both tables.
After about 25 minutes (on 4 processor Del 6300 with 1GB Memory) it
spits out:

ERROR: Write to hashjoin temp file failed

tiger=# explain select * from bk_inv i, bk_title t where i.isbn = t.isbn
and t.vendor ='50000029';
NOTICE: QUERY PLAN:

Merge Join (cost=0.00..11229637.06 rows=2172466 width=72)
-> Index Scan using bk_title_isbn_idx on bk_title t
(cost=0.00..390788.08 rows=107331 width=24)
-> Index Scan using bk_inv_isbn_idx on bk_inv i
(cost=0.00..10252621.38 rows=46790877 width=48)

BIG COST!

These explain queries show the existance of the indexes and give small
costs:

tiger=# explain select * from bk_title where isbn = '50000029';
NOTICE: QUERY PLAN:

Index Scan using bk_title_isbn_idx on bk_title (cost=0.00..4.90 rows=1
width=24)

tiger=# explain select * from bk_inv where isbn = '0897474228';
NOTICE: QUERY PLAN:

Index Scan using bk_inv_isbn_idx on bk_inv (cost=0.00..225.53 rows=55
width=48)

Note. Same tables, same query returns instantaneously with Oracle 8.1.
What I am hoping to show is that Postgres can do our job too.

Any help on this much obliged. (Yes I ran vacuum analyze).

David Link
White Plains, NY