Eliminate N+1 Queries with Batch Loading#
N+1 queries execute one query per item in a loop. Batch them into a single query using arrays or JOINs.
Incorrect (N+1 queries):
-- First query: get all users
select id from users where active = true; -- Returns 100 IDs
-- Then N queries, one per user
select * from orders where user_id = 1;
select * from orders where user_id = 2;
select * from orders where user_id = 3;
-- ... 97 more queries!
-- Total: 101 round trips to database
Correct (single batch query):
-- Collect IDs and query once with ANY
select * from orders where user_id = any(array[1, 2, 3, ...]);
-- Or use JOIN instead of loop
select u.id, u.name, o.*
from users u
left join orders o on o.user_id = u.id
where u.active = true;
-- Total: 1 round trip
Application pattern:
-- Instead of looping in application code:
-- for user in users: db.query("SELECT * FROM orders WHERE user_id = $1", user.id)
-- Pass array parameter:
select * from orders where user_id = any($1::bigint[]);
-- Application passes: [1, 2, 3, 4, 5, ...]
Reference: N+1 Query Problem