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+---
+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):**
+
+```sql
+-- 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):**
+
+```sql
+-- 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:
+
+```sql
+-- 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](https://supabase.com/docs/guides/database/query-optimization)