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I've got a simple recursive query to find a tree of all posts:

with recursive all_mentions(post_id, author_id, mentioned_id) as (
  select post_id, author_id, mention_id
  from mentions
  where author_id = 'abc123' -- <-- index gets used

  union all

  select next.post_id, next.author_id, next.mention_id
  from all_mentions as cur
  inner join mentions as next on
    next.author_id = cur.mention_id -- <-- index does not get used
  where next.post_id > cur.post_id
)
select *
from all_mentions

There is an index on mentions.author_id, which gets used in the first portion of the query (ie, for … where author_id = 'abc123'), but it does not get used in the second part of the query (ie, … where next.author_id = cur.mention_id), and instead there is a table scan + sort of the 500M line table:

Limit  (cost=3818225300.80..3818225310.80 rows=500 width=136)
  CTE all_mentions
    ->  Recursive Union  (cost=779.32..3818225300.80 rows=13002675849 width=193)
          ->  Bitmap Heap Scan on mentions  (cost=779.32..75336.89 rows=18919 width=193)
                Recheck Cond: (("author_id")::citext = '0xc5918a927c4fb83fe99e30d6f66707f4b396900e'::citext)
USES INDEX =>   ->  Bitmap Index Scan on mentions_author_id  (cost=0.00..774.59 rows=18919 width=0)
                      Index Cond: (("author_id")::citext = '0xc5918a927c4fb83fe99e30d6f66707f4b396900e'::citext)
          ->  Merge Join  (cost=282994142.90..355809644.69 rows=1300265693 width=193)
                Merge Cond: ((all_mentions_1."author_id")::citext = (next."mention_id")::citext)
                Join Filter: (next.post_id > all_mentions_1.post_id)
                ->  Sort  (cost=25542.31..26015.28 rows=189190 width=40)
                      Sort Key: all_mentions_1."author_id"
                      ->  WorkTable Scan on all_mentions all_mentions_1  (cost=0.00..3783.80 rows=189190 width=40)
                ->  Materialize  (cost=282968600.59..285568486.01 rows=519977084 width=166)
                      ->  Sort  (cost=282968600.59..284268543.30 rows=519977084 width=166)
DOES NOT USE INDEX =>       Sort Key: next."mention_id"
                            ->  Seq Scan on mentions next  (cost=0.00..37074366.84 rows=519977084 width=166)
  ->  CTE Scan on all_mentions  (cost=0.00..260053516.98 rows=13002675849 width=136)

Other relevant details:

  • There are many (millions) authors, and each author has a relatively small number of posts (on ~10-100; obviously there's a long tail)
  • I expect the query to return ~hundreds of rows

Any pointers towards why this could be?

  • Please edit your question (by clicking on the edit link below your question) and show us the execution plan generated using explain (analyze, buffers) not just a "simple" explain. – a_horse_with_no_name Oct 8 '18 at 17:04
  • @a_horse_with_no_name the query would take ~hours to run with analyze, which is why I haven't included the output (verified by letting it run for ~20 minutes before stopping it.) – David Wolever Oct 15 '18 at 17:47

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