bug5.txt

1. Grab the DB dump

curl -L "https://www.dropbox.com/s/k1ai0765gc2k98f/bug5.sql?dl=1" -o bug5.sql

2. Create an empty database and restore the dump and wait a bit:

createdb bug5 && psql -d bug5 -f bug5.sql && psql -d bug5 -c "ANALYZE"

3. Run queries:

psql -d bug5 -c "set enable_indexscan = 'off'; SELECT a.id FROM a JOIN b ON b.a_id = a.id JOIN c ON c.id = b.c_id WHERE c.tag = '13880'"

1 row expected, get 1

psql -d bug5 -c "set enable_indexscan = 'on'; SELECT a.id FROM a JOIN b ON b.a_id = a.id JOIN c ON c.id = b.c_id WHERE c.tag = '13880'"

1 row expected, get 0

If it doesn't reproduce, drop the DB and try again 

dropdb bug5