Problem with a view (not lazy ;)

Hi all.

In my tests with views I have found a problem that can't explain.

Look at this view:
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create view v_ingresos_técnico as
select e.nombre as empleado, p.importe_total, p.iva, p.num_factura,
p.fecha_factura, p.cobrado, a.urgente, ta.fecha
from técnico_aviso ta
join empleado e on ta.empleado = e.cod
join presupuesto p on ta.aviso = p.aviso
join aviso a on ta.aviso = a.número;
--------------------------------------

When I query:

SELECT empleado, sum(importe_total) from v_ingresos_técnico
where fecha between '1/1/2002' and '31/1/2002' group by
empleado;

It executes inmediately. Here is the explain:
--------------------------------------
Aggregate (cost=2930.48..2930.48 rows=1 width=40)
-> Group (cost=2930.48..2930.48 rows=1 width=40)
-> Sort (cost=2930.48..2930.48 rows=1 width=40)
-> Nested Loop (cost=3.71..2930.47 rows=1 width=40)
-> Nested Loop (cost=3.71..2506.35 rows=177 width=36)
-> Hash Join (cost=3.71..1449.89 rows=510 width=24)
-> Seq Scan on técnico_aviso ta (cost=0.00..1424.06 rows=510 width=8)
-> Hash (cost=3.37..3.37 rows=137 width=16)
-> Seq Scan on empleado e (cost=0.00..3.37 rows=137 width=16)
-> Index Scan using pre_aviso_ndx on presupuesto p (cost=0.00..2.06 rows=1 width=12)
-> Index Scan using aviso_pkey on aviso a (cost=0.00..2.39 rows=1 width=4)
--------------------------------------

Now look at this view. Differences with previous are marked with "->":
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create view v_ingresos_técnico as
select e.nombre as empleado, p.importe_total, p.iva, p.num_factura,
p.fecha_factura, p.cobrado, a.urgente, ta.fecha,
-> em.descripcion as empresa
from técnico_aviso ta
join empleado e on ta.empleado = e.cod
join presupuesto p on ta.aviso = p.aviso
join aviso a
-> left join empresa em on a.empresa = em.cod
on ta.aviso = a.número;
--------------------------------------

Executing same query it lags for more than a minute. Here is explain:
--------------------------------------
Aggregate (cost=5679.37..5679.38 rows=1 width=46)
-> Group (cost=5679.37..5679.37 rows=1 width=46)
-> Sort (cost=5679.37..5679.37 rows=1 width=46)
-> Nested Loop (cost=1.34..5679.36 rows=1 width=46)
-> Nested Loop (cost=0.00..6.28 rows=1 width=36)
-> Nested Loop (cost=0.00..4.21 rows=1 width=24)
-> Index Scan using tec_avi_fecha_ndx on técnico_aviso ta (cost=0.00..2.18 rows=1 width=8)
-> Index Scan using empleado_pkey on empleado e (cost=0.00..2.01 rows=1 width=16)
-> Index Scan using pre_aviso_ndx on presupuesto p (cost=0.00..2.06 rows=1 width=12)
-> Materialize (cost=4974.88..4974.88 rows=46547 width=10)
-> Merge Join (cost=1.34..4974.88 rows=46547 width=10)
-> Index Scan using avi_empresa_ndx on aviso a (cost=0.00..4391.56 rows=46547 width=6)
-> Sort (cost=1.34..1.34 rows=12 width=4)
-> Seq Scan on empresa em (cost=0.00..1.12 rows=12 width=4)
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I am curious about "Materialize" but don't know what it means.

Table "empresa" has 12 rows and there are indexes for all fields that
participate in a join.

Please, any help with this problem?.

Thanks in advance.

David