**Measuring Query Performance:**

```sql

-- PostgreSQL: EXPLAIN ANALYZE shows actual execution time

EXPLAIN ANALYZE

SELECT o.order_id, c.customer_name, o.total_amount

FROM orders o

JOIN customers c ON o.customer_id = c.customer_id

WHERE o.order_date >= '2024-01-01';

-- MySQL: EXPLAIN shows execution plan

EXPLAIN FORMAT=JSON

SELECT o.order_id, c.customer_name, o.total_amount

FROM orders o

JOIN customers c ON o.customer_id = c.customer_id

WHERE o.order_date >= '2024-01-01';

-- SQL Server: Include actual execution plan

SET STATISTICS TIME ON;

SET STATISTICS IO ON;

SELECT o.order_id, c.customer_name, o.total_amount

FROM orders o

JOIN customers c ON o.customer_id = c.customer_id

WHERE o.order_date >= '2024-01-01';

```

**Reading Execution Plans:**

Key metrics to look for:

- **Seq Scan / Table Scan**: Full table scan (bad for large tables)

- **Index Scan**: Using an index (good)

- **Index Seek**: Direct index lookup (best)

- **Nested Loop**: Good for small datasets

- **Hash Join**: Good for large datasets

- **Rows**: Estimated vs actual row counts

- **Cost**: Relative cost estimate

- **Time**: Actual execution time

**Common Plan Patterns:**

```

Good Plan:

-> Index Seek (cost=0.43 rows=10)

-> Nested Loop Join (cost=15.2 rows=10)

-> Index Scan (cost=8.5 rows=100)

Bad Plan:

-> Seq Scan (cost=10000 rows=1000000)