query with offset stops using index scan

Hello everybody,

I have some weird behaviour with a pretty simple query, which I use in a web
front end to browse through pages of data.

SELECT

foo.id, get_processing_status(foo.id) AS status, foo.name,
foo.valid_until

FROM

foo

WHERE foo.active AND foo.valid_until < 1220186528 AND NOT foo.locked

ORDER BY foo.id DESC

LIMIT 25

OFFSET 100

This very query works quite quickly, and the query plan looks like this:

"Limit (cost=36.04..45.05 rows=25 width=63)"

" -> Index Scan Backward using foo_pkey on foo (cost=0.00..511.35
rows=1419 width=63)"

" Filter: (active AND (valid_until < 1220186528) AND (NOT locked))"

Now, if I increase OFFSET slowly, it works all the same way, until OFFSET
reaches the value of 750. Then, the planner refuses to use an index scan and
does a plain seq scan+sort, which makes the query about 10-20 times slower:

"Limit (cost=272.99..273.05 rows=25 width=63)"

" -> Sort (cost=271.11..274.66 rows=1419 width=63)"

" Sort Key: id"

" -> Seq Scan on foo (cost=0.00..196.82 rows=1419 width=63)"

" Filter: (active AND (valid_until < 1220186528) AND (NOT
locked))"

I use 8.1.4, and I did a vacuum full analyze before running the queries.

What might be the issue here? Could a reindex on the pkey help?

Kind Regards

Stanisalv Raskin