nextval skips values between consecutive calls

I just found an odd thing about nextval (PostgreSQL 9.0): When nextval is called together with a function returning a sequence, such as generate_series or unnest, it skips one value between consecutive calls:

create sequence test_sequence;

-- This works as expected
select nextval(' test_sequence'); -- 1
select nextval(' test_sequence'); -- 2

-- This is rather surprising
select nextval(' test_sequence'), generate_series(1, 1); -- 3, 1
select nextval(' test_sequence'), generate_series(1, 1); -- 5, 1

drop sequence test_sequence;

Is there any explanation for why nextval skips a value in the second case?

By the way, if the second query is rewritten as follows, nextval again generates consecutive values:

select nextval(' test_sequence'), ind
from (select generate_series(1, 1) ind) A;

Is this a bug or a feature?

Dmitry Epstein | Developer

Allied Testing

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