cost difference for 2 similar queries

Hi,

I have 2 queries, which are quite similar:

a) select distinct p1.domain from dom_prot p1, identical p2, classification p3
where p2.reference = p1.protein
and p2.protein ~ 'EPA3_HUMAN'
and p3.domain = p1.domain;

b) select distinct p1.domain from dom_prot p1, identical p2, classification p3
where p2.reference = p1.protein
and p2.protein ~ 'EPA3_HUMAN'
and p3.domain = p1.domain
and p3.class = 'extra';

As you see, they differ only in the last line. The problem is, that the time
they need differes tremendously. Query a) takes about 5 sec, query b) 2 min !!
I don't understand why this last constraint over an indexed field leads to such
a difference.

Here is what explain says:

a)
NOTICE: QUERY PLAN:

Unique (cost=5193.21 size=0 width=0)
-> Sort (cost=5193.21 size=0 width=0)
-> Nested Loop (cost=5193.21 size=1 width=48)
-> Nested Loop (cost=5191.16 size=1 width=36)
-> Seq Scan on p2 (cost=5189.11 size=1 width=12)
-> Index Scan on p1 (cost=2.05 size=62250 width=24)
-> Index Scan on p3 (cost=2.05 size=189 width=12)
EXPLAIN

b)
NOTICE: QUERY PLAN:

Unique (cost=680.60 size=0 width=0)
-> Sort (cost=680.60 size=0 width=0)
-> Nested Loop (cost=680.60 size=1 width=48)
-> Nested Loop (cost=4.10 size=330 width=36)
-> Index Scan on p3 (cost=2.05 size=1 width=12)
-> Index Scan on p1 (cost=2.05 size=62250 width=24)
-> Index Scan on p2 (cost=2.05 size=1 width=12)

EXPLAIN

Dou you have any idea, how to improve query b) ??

Thanx,
Joerg

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