Self-join optimisation

Hi,

I asked this question in the Postgres Slack, and was recommended to ask
here instead.

A few times, I've been in a situation where I want to join a table to
itself on its primary key. That typically happens because I have some kind
of summary view, which I then want to join to the original table (using its
primary key) to flesh out the summary data with other columns. That's
executed as a join, which surprised me. But in this case, I could extend
the view to have all of the columns of the original table to avoid the join.

But there's another case that's harder to solve this way: combining views
together. Here's a trivial example:

CREATE TABLE users (id BIGINT PRIMARY KEY, varchar name);
CREATE VIEW only_some_users AS (SELECT * FROM users WHERE id < 10);
CREATE VIEW some_other_users AS (SELECT * FROM users WHERE id > 3);

EXPLAIN SELECT * FROM only_some_users
INNER JOIN some_other_users ON only_some_users.id = some_other_users.id;

Hash Join (cost=29.23..43.32 rows=90 width=144)

Hash Cond: (users.id = users_1.id)

-> Bitmap Heap Scan on users (cost=6.24..19.62 rows=270 width=72)

Recheck Cond: (id < 10)

-> Bitmap Index Scan on users_pkey (cost=0.00..6.18 rows=270
width=0)
Index Cond: (id < 10)

-> Hash (cost=19.62..19.62 rows=270 width=72)

-> Bitmap Heap Scan on users users_1 (cost=6.24..19.62 rows=270
width=72)
Recheck Cond: (id > 3)

-> Bitmap Index Scan on users_pkey (cost=0.00..6.18
rows=270 width=0)
Index Cond: (id > 3)

Is there a reason why Postgres doesn't have an optimisation built in to
optimise this JOIN? What I'm imagining is that a join between two aliases
for the same table on its primary key could be optimised by treating them
as the same table. I think the same would be true for self-joins on any
non-null columns covered by a uniqueness constraint.

If this is considered a desirable change, I'd be keen to work on it (with
some guidance).

Thanks,

Hywel
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