I have the following schema:
CREATE TABLE users ( id BIGSERIAL PRIMARY KEY, name TEXT NOT NULL UNIQUE ); CREATE TABLE friends ( user_id BIGINT NOT NULL REFERENCES users, friend_user_id BIGINT NOT NULL REFERENCES users, UNIQUE (user_id, friend_user_id) ); CREATE TABLE posts ( id BIGSERIAL PRIMARY KEY, user_id BIGINT NOT NULL REFERENCES users, content TEXT NOT NULL ); CREATE INDEX posts_user_id_id_index ON posts(user_id, id);
Each user can unilaterally follow any number of friends. The
posts table has a large number of rows and is rapidly growing.
My goal is to retrieve the 10 most recent posts of a user's friends. This query gives the correct result, but is inefficient:
SELECT posts.id, users.name, posts.content FROM posts JOIN users ON posts.user_id = users.id WHERE posts.user_id IN (SELECT friend_user_id FROM friends WHERE user_id = 1) ORDER BY posts.id DESC LIMIT 10;
If the user's friends have recently posted, the query is still reasonably fast (query plan). But if the user's friends haven't recently posted or the user has no friends, it quickly deteriorates (query plan).
If I match only a single post author (e.g.
WHERE posts.user_id = 5), Postgres uses the index
posts_user_id_id_index. But if I use
IN, the index doesn't appear to be used at all.
How can I get these results more efficiently?
I've uploaded the schema and the queries I've tried to dbfiddle. The output of
SELECT version() is
PostgreSQL 9.6.5 on x86_64-pc-linux-gnu, compiled by gcc (Debian 4.9.2-10) 4.9.2, 64-bit for me.