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+---
+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):**
+
+```sql
+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):**
+
+```sql
+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:
+
+```sql
+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](https://www.postgresql.org/docs/current/ddl-constraints.html#DDL-CONSTRAINTS-FK)