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, 100);
create index numbers_n_idx on numbers(n);
-- Non-indexed queries
explain analyze select numbers.n from (values (500)) quantities(q)
join numbers on numbers.n in (select q);
explain analyze select numbers.n from (values (500)) 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!
repro.sh
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