Schema Foreign Key Indexes

title: Index Foreign Key Columns impact: HIGH impactDescription: 10-100x faster JOINs and CASCADE operations tags: foreign-key, indexes, joins, schema

Index Foreign Key Columns

Postgres does not automatically index foreign key columns. Missing indexes cause slow JOINs and CASCADE operations.

Incorrect (unindexed foreign key):

create table orders (
  id bigint generated always as identity primary key,
  customer_id bigint references customers(id) on delete cascade,
  total numeric(10,2)
);

-- No index on customer_id!
-- JOINs and ON DELETE CASCADE both require full table scan
select * from orders where customer_id = 123;  -- Seq Scan
delete from customers where id = 123;          -- Locks table, scans all orders

Correct (indexed foreign key):

create table orders (
  id bigint generated always as identity primary key,
  customer_id bigint references customers(id) on delete cascade,
  total numeric(10,2)
);

-- Always index the FK column
create index orders_customer_id_idx on orders (customer_id);

-- Now JOINs and cascades are fast
select * from orders where customer_id = 123;  -- Index Scan
delete from customers where id = 123;          -- Uses index, fast cascade

Find missing FK indexes:

select
  conrelid::regclass as table_name,
  a.attname as fk_column
from pg_constraint c
join pg_attribute a on a.attrelid = c.conrelid and a.attnum = any(c.conkey)
where c.contype = 'f'
  and not exists (
    select 1 from pg_index i
    where i.indrelid = c.conrelid and a.attnum = any(i.indkey)
  );

Reference: Foreign Keys