supabase-postgres-best-practices/references/query-missing-indexes.md

title: Add Indexes on WHERE and JOIN Columns impact: CRITICAL impactDescription: 100-1000x faster queries on large tables tags: indexes, performance, sequential-scan, query-optimization

Add Indexes on WHERE and JOIN Columns

Queries filtering or joining on unindexed columns cause full table scans, which become exponentially slower as tables grow.

Incorrect (sequential scan on large table):

-- No index on customer_id causes full table scan
select * from orders where customer_id = 123;

-- EXPLAIN shows: Seq Scan on orders (cost=0.00..25000.00 rows=100 width=85)

Correct (index scan):

-- Create index on frequently filtered column
create index orders_customer_id_idx on orders (customer_id);

select * from orders where customer_id = 123;

-- EXPLAIN shows: Index Scan using orders_customer_id_idx (cost=0.42..8.44 rows=100 width=85)

For JOIN columns, always index the foreign key side:

-- Index the referencing column
create index orders_customer_id_idx on orders (customer_id);

select c.name, o.total
from customers c
join orders o on o.customer_id = c.id;

Reference: Query Optimization