On Thu, 11 Mar 2021 at 15:15, Hywel Carver <hy...@skillerwhale.com> wrote: > > 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).
There's currently a patch registered in the commitfest that could fix this for you, called "Remove self join on a unique column" [0]. With regards, Matthias van de Meent [0] https://commitfest.postgresql.org/31/1712/, thread at https://www.postgresql.org/message-id/flat/64486b0b-0404-e39e-322d-080115490...@postgrespro.ru
