Hey y'all! So recently, I ran into an issue where a query I wrote wasn't using an index, presumably because what I was doing was too hard for the query planner to figure out. I've distilled the problem into its essence to the best of my ability, and found that it's because `select` seems to hinder it. The problem boils down to the planner not figuring out that these two queries should use an index: ```sql -- Setup create table numbers(n int); insert into numbers (n) select generate_series(1, 1000000); create index numbers_n_idx on numbers(n); -- Non-indexed queries explain analyze select numbers.n from (values (5000000)) quantities(q) join numbers on numbers.n in (select q); explain analyze select numbers.n from (values (5000000)) quantities(q) join numbers on numbers.n = any(select q); ``` These examples may seem silly, so let me provide a "case study" query that should justify the need for such an optimization. I had a query that was generating an array of items, and wanted to join it to a table given that some column of that table was present in the array. It looked like so: ```sql select numbers.n from quantities join numbers on numbers.n in (select unnest(quantities.q)); ``` This turned out to be horrendously slow, because it was performing a sequential scan! I did however end up settling on the following form: ```sql select numbers.n from quantities join numbers on numbers.n = any(quantities.q); ``` This was only possible because I was dealing with arrays though, and an operation such as `in (select unnest...)` can be easily converted to `= any(...)`. However for the general case, I believe an optimization in this area may provide benefit as there may exist a circumstance that does not have an alternative to a sub-query select (`= any()` was my alternative), but I am just a database newbie. I've noticed this problem has been around since at least 11.7, and is still present as of the `postgres:15beta1` docker image. I've attached a script which reproduces the issue. It uses docker, so I'm confident you'll be able to run it without issue. Finally, I ask: - Is this an issue that should be fixed? I'm a database newbie so I have no idea about the deep semantics of SQL and what a select inside a `join_condition` could imply to the planner to prevent it from optimizing it. - If "yes" to the previous question, what would be the precise semantics of such an optimization? I loosely say `n in (select q)` -> `n in (q)` for all n and q, but of course I don't have enough knowledge to know that this is correct in terms of whatever Postgres' internal query optimization IR is. - Can a database newbie like myself contribute an optimization pass in Postgres to fix this? I'm fascinated by the work y'all do, and submitting a patch to Postgres that makes it into production would make my week. Thank you for your time, and have a great day!
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