Hi and thank you for the response.
I tried VACUUM ANALYZE for three tables, but without success. I also tried to set enable_seqscan=off and the query took even more time. If I set enable_sort=off then the query takes a lot of time and I cancel it.
Maybe you could restore (to a temp server, not the production) a physical backup taken from before the change happened, and get an old plan that way. I'm guessing that somehow an index got dropped around the same time you took the dump. That might be a lot of work, and maybe it would just be easier to optimize the current query while ignoring the past. But you seem to be interested in a root-cause analysis, and I don't see any other way to do one of those.
What I would expect to be the winning plan would be something sort-free like:
Limit
merge join
index scan yielding books in asin order (already being done)
nested loop
index scan yielding asins in value order
index scan probing asins_statistics driven by asins_statistics.asin_id =
asins.id
Or possibly a 2nd nested loop rather than the merge join just below the limit, but with the rest the same
In addition to the "books" index already evident in your current plan, you would also need an index leading with asins_statistics.asin_id, and one leading with asins.value. But if all those indexes exists, it is hard to see why setting enable_seqscan=off wouldn't have forced them to be used.
Cheers,
Jeff