Query > 1000× slowdown after adding datetime comparison

I have the following three tables:

DOCUMENT
  id (index)
  documenttype
  date_last_updated: timestamp(6) (indexed)

EXTERNAL_TRANSLATION_UNIT
  id (indexed)
  fk_id_document (indexed)

EXTERNAL_TRANSLATION
  id (indexed)
  fk_id_translation_unit (indexed)

Table sizes:
 DOCUMENT: 381 000
 EXTERNAL_TRANSLATION_UNIT: 76 000 000
 EXTERNAL_TRANSLATION: 76 000 000

Now the following query takes about 36 minutes to finish:

 SELECT u.id AS id_external_translation_unit,
    r.id AS id_external_translation,
    u.fk_id_language AS fk_id_source_language,
    r.fk_id_language AS fk_id_target_language,
    doc.fk_id_job
 FROM "EXTERNAL_TRANSLATION_UNIT" u
     JOIN "DOCUMENT" doc ON u.fk_id_document = doc.id
     JOIN "EXTERNAL_TRANSLATION" r ON u.id = r.fk_id_translation_unit
 WHERE doc.date_last_updated >= date(now() - '171:00:00'::interval)
 ORDER BY r.id LIMIT 1000

This is the query plan:

<http://postgresql.nabble.com/file/n5864045/qp1.png>

If I remove the WHERE condition, it returns immediately.

Am I doing something obviously wrong?

Thank you for any ideas.



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