mysterious difference in speed when combining two queries with OR

I cannot understand why the following two queries differ so much in execution time (almost ten times)

Query A (two queries)

select distinct moment.mid from moment,timecard where parent = 45 and (pid=17 and timecard.mid = moment.mid) order by
moment.mid;
select distinct moment.mid from moment,timecard where parent = 45 and (pbar = 0) order by moment.mid;

Query B (combining the two with OR)

select distinct moment.mid from moment,timecard where parent = 45 and ((pid=17 and timecard.mid = moment.mid) or (pbar
=0)) order by moment.mid; 

$ time psql -o /dev/null -f query-a.sql fektest

real    0m2.016s
user    0m1.532s
sys     0m0.140s

$ time psql -o /dev/null -f query-b.sql fektest

real    0m28.534s
user    0m1.516s
sys     0m0.156s

I have tested this in two different computers with different amount of
RAM, fast or slow CPU, and the difference is persistent, almost ten
times.

I should say that this is on postgresql 7.4.16 (debian stable).

Can query B be rewritten so that it would execute faster?

TIA

--
Hans Ekbrand (http://sociologi.cjb.net) <hans@sociologi.cjb.net>
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