Explain output: wrong row count?

I have this EXPLAIN ANALYZE output:

alvherre=# explain analyze select * from oficina join ciudad using
(codciudad) where codtipoofi in (4,5);                                                   QUERY PLAN
                              
 
------------------------------------------------------------------------------------------------------------------Hash
Join (cost=15.24..68.76 rows=621 width=394) (actual time=6.003..22.175 rows=641 loops=1)  Hash Cond: ("outer".codciudad
="inner".codciudad)  ->  Seq Scan on oficina  (cost=0.00..42.66 rows=621 width=309) (actual time=0.013..3.103 rows=641
loops=1)       Filter: ((codtipoofi = 4) OR (codtipoofi = 5))  ->  Hash  (cost=13.99..13.99 rows=499 width=89) (actual
time=5.947..5.947rows=0 loops=1)        ->  Seq Scan on ciudad  (cost=0.00..13.99 rows=499 width=89) (actual
time=0.018..3.909rows=499 loops=1)Total runtime: 24.448 ms
 
(7 rows)

Note that the Hash step has an estimated row count of 499 (which is a
good estimate), but the actual row count is 0, which is way off.  But,
the outer Hash Join step has a good estimate.

I wonder if the rows=0 is a bug, or is that number meant to be
interpreted in some special way?  It is the same on both 7.4.6 and
8.0beta5.  (Now that I look, it's 0 in all Hash steps I have at sight
... I had never noticed before!)


This query doesn't actually affect me a lot, but it is part of a bigger
query whose estimation is way off.  I won't post it here because it's
topic for pgsql-performance ...

-- 
Alvaro Herrera (<alvherre[@]dcc.uchile.cl>)
"No reniegues de lo que alguna vez creíste"