Re: Self-join optimisation

On Thu, 11 Mar 2021 at 15:15, Hywel Carver <hywel(at)skillerwhale(dot)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-0801154901f3(at)postgrespro(dot)ru