ORDER BY performance deteriorates very quickly as dataset grows

I have a model like this:

http://i.stack.imgur.com/qCZpD.png

with approximately these table sizes

JOB: 8k
DOCUMENT: 150k
TRANSLATION_UNIT: 14,5m
TRANSLATION: 18,3m

Now the following query takes about 90 seconds to finish.

*select* translation.id
*from* "TRANSLATION" translation
*inner join* "TRANSLATION_UNIT" unit
*on* translation.fk_id_translation_unit = unit.id
*inner join* "DOCUMENT" document
*on* unit.fk_id_document = document.id
*where* document.fk_id_job = 11698
*order by* translation.id *asc*
*limit* 50 *offset* 0

Query plan: http://explain.depesz.com/s/xlR

With the following modification, the time is reduced to 20-30 seconds (query
plan <http://explain.depesz.com/s/VkI>)

*with* CTE *as* (
*select* tr.id
*from* "TRANSLATION" tr
*inner join *"TRANSLATION_UNIT" unit
*on* tr.fk_id_translation_unit = unit.id
*inner join* "DOCUMENT" doc
*on* unit.fk_id_document = doc.id
*where* doc.fk_id_job = 11698)
*select* * *from *CTE
*order by* id *asc*
*limit* 50 *offset* 0;

There are about 212,000 records satisfying the query's criteria. When I
change 11698 to another id in the query so that there are now cca 40,000
matching records, the queries take 40ms and 55ms, respectively. The query
plans also change: the original query <http://explain.depesz.com/s/cDT>, the
CTE variant <http://explain.depesz.com/s/9ow>.

Is it normal to experience 2100× increase in the execution time (or cca 450×
for the CTE variant) when the number of matching records grows just 5 times?

I ran *ANALYZE* on all tables just before executing the queries. Indexes
are on all columns involved.

System info:

PostgreSQL 9.2

shared_buffers = 2048MB
effective_cache_size = 4096MB
work_mem = 32MB

Total memory: 32GB
CPU: Intel Xeon X3470 @ 2.93 GHz, 8MB cache

Thank you.