-- Read Committed (default)
BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;

-- Snapshot Isolation
BEGIN TRANSACTION ISOLATION LEVEL SNAPSHOT;
SELECT * FROM accounts;  -- Consistent view throughout transaction
COMMIT;

-- Query data as it existed at a specific timestamp
SELECT * FROM orders AS OF TIMESTAMP '2024-01-15 10:30:00';

-- Query data as of a specific transaction
SELECT * FROM inventory AS OF TRANSACTION 1234;

-- Compare current vs historical data
SELECT
    current.price,
    historical.price AS old_price
FROM products current
JOIN products AS OF TIMESTAMP '2024-01-01' historical
    ON current.id = historical.id
WHERE current.price != historical.price;

-- B-tree: Range queries, sorting, prefix matching
CREATE INDEX idx_date ON orders(created_at) USING BTREE;
SELECT * FROM orders WHERE created_at BETWEEN '2024-01-01' AND '2024-12-31';

-- Hash: O(1) equality lookups
CREATE INDEX idx_email ON users(email) USING HASH;
SELECT * FROM users WHERE email = '[email protected]';

-- Bitmap: Low-cardinality columns, efficient AND/OR
CREATE INDEX idx_status ON orders(status) USING BITMAP;
SELECT * FROM orders WHERE status = 'pending' AND priority = 'high';

-- Multi-column composite indexes
CREATE INDEX idx_lookup ON events(user_id, event_type, created_at);
SELECT * FROM events WHERE user_id = 100 AND event_type = 'click';

SELECT
    employee_name,
    department,
    salary,
    ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) as rank,
    salary - LAG(salary) OVER (ORDER BY hire_date) as salary_change,
    AVG(salary) OVER (PARTITION BY department) as dept_avg,
    SUM(salary) OVER (ORDER BY hire_date ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) as running_total
FROM employees;

-- Non-recursive CTE
WITH high_value_orders AS (
    SELECT * FROM orders WHERE amount > 1000
)
SELECT customer_id, COUNT(*) FROM high_value_orders GROUP BY customer_id;

-- Recursive CTE (e.g., organizational hierarchy)
WITH RECURSIVE org_chart AS (
    SELECT id, name, manager_id, 1 as level
    FROM employees WHERE manager_id IS NULL

    UNION ALL

    SELECT e.id, e.name, e.manager_id, oc.level + 1
    FROM employees e
    JOIN org_chart oc ON e.manager_id = oc.id
)
SELECT * FROM org_chart ORDER BY level, name;

-- ROLLUP: Hierarchical subtotals
SELECT region, product, SUM(sales)
FROM sales_data
GROUP BY ROLLUP(region, product);

-- CUBE: All possible subtotal combinations
SELECT region, product, SUM(sales)
FROM sales_data
GROUP BY CUBE(region, product);

-- GROUPING SETS: Specific grouping combinations
SELECT region, product, category, SUM(sales)
FROM sales_data
GROUP BY GROUPING SETS ((region, product), (category), ());

-- Correlated subquery
SELECT * FROM employees e
WHERE salary > (SELECT AVG(salary) FROM employees WHERE department = e.department);

-- EXISTS
SELECT * FROM customers c
WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.id AND o.amount > 1000);

-- IN with subquery
SELECT * FROM products
WHERE category_id IN (SELECT id FROM categories WHERE active = true);

-- Collect table statistics
ANALYZE orders;

-- View query execution plan
EXPLAIN SELECT * FROM orders WHERE customer_id = 100;

-- View plan with actual execution statistics
EXPLAIN ANALYZE SELECT * FROM orders o
JOIN customers c ON o.customer_id = c.id
WHERE c.country = 'US';