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I have a query which has a huge bottleneck. Doing explain analyze I found out that an index scan is causing the problem.

The query is:

select * from product where (product.id = IN(select pu.id from product_user pu where pu.userid = 123)) limit 40

So the mytable.id = IN() causes an index scan which is really slow. After some searching on the internet I found that rewriting the query like this..

select * from product where (product.id = ANY((SELECT ARRAY(select pu.id from product_user pu where pu.userid = 123)::bigint[]))) limit 40

..performs way better, like 70% faster. Please note that the subquery (standalone) is super fast, but does return like 300k records.

Also, adding a manual btree index on the primary key (ID) column of the product table does increase performance a bit as well. Not sure why this is faster than the default primary key index.

Here's the full original query + query plan: https://explain.depesz.com/s/UW1w > Here's the same a bit more readable with a few strings translated: https://explain.depesz.com/s/Ry99#query

Here's the one with ANY(SELECT ARRAY.. instead of IN: https://explain.depesz.com/s/DS33

The query is generated by third party software. I have almost no influence on this query. Is there any way to either:

  1. Tune postgres so the index scan from the IN() subquery will be faster
  2. Rewrite a query right before execution on database level. We host the postgres database in the Google Cloud (managed databases), some kind of interceptor, does it exist?

Additional info:

  • Postgres 13
  • Total size of mytable: 9.000.000 rows
  • Total size of subquery: +-300.000 rows

Any tips and suggestions are very welcome! Thanks!

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  • Please consider reading this advice
    – mustaccio
    May 23, 2023 at 11:44
  • Thanks, added more info and full query + explain info
    – Marcel
    May 23, 2023 at 11:59
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    FWIW, a simple JOIN between product and product_user would probably be the best way to re-write it. Unfortunately for you, re-writing the query sounds unlikely. The best outcome would come from convincing the 3rd party software to fix their crappy code though.
    – J.D.
    May 23, 2023 at 12:10
  • Yeah, that's going to be a problem :) There is actually some login behind why the query is this way, but in certain scenarios it's just super inefficient. Altough I also don't get why Postgres is this inefficient in chosing the best plan. As I turn off "enable_nestloop" for example the query is way faster.
    – Marcel
    May 23, 2023 at 12:53
  • @Marcel Long IN lists as part of a predicate are a common problem for most RDBMS - it's not a PostgreSQL specific problem. They are the equivalent of a bunch of OR clauses. It just complicates the query too much. I actually consider them an anti-pattern. I can't think of a single reason where your example query would be any different as a JOIN though, btw.
    – J.D.
    May 23, 2023 at 13:30

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