Denormalisering

-- NORMALIZED design (3NF)
CREATE TABLE users (
  user_id INT PRIMARY KEY,
  username VARCHAR(50),
  email VARCHAR(100)
);

CREATE TABLE posts (
  post_id INT PRIMARY KEY,
  user_id INT REFERENCES users(user_id),
  title VARCHAR(200),
  content TEXT,
  created_at TIMESTAMP
);

-- Query kræver JOIN:
SELECT p.title, p.content, u.username
FROM posts p
JOIN users u ON p.user_id = u.id
WHERE p.post_id = 123;

-- DENORMALIZED design
CREATE TABLE posts_denorm (
  post_id INT PRIMARY KEY,
  user_id INT REFERENCES users(user_id),
  username VARCHAR(50),  -- REDUNDANT! Also in users table
  title VARCHAR(200),
  content TEXT,
  created_at TIMESTAMP
);

-- Query UDEN JOIN:
SELECT title, content, username
FROM posts_denorm
WHERE post_id = 123;
-- Meget hurtigere!

-- Pattern 1: Storing aggregates
-- NORMALIZED:
CREATE TABLE posts (
  post_id INT PRIMARY KEY,
  title VARCHAR(200)
);

CREATE TABLE comments (
  comment_id INT PRIMARY KEY,
  post_id INT REFERENCES posts,
  text TEXT
);

-- Count comments (slow med mange comments):
SELECT p.*, COUNT(c.comment_id) as comment_count
FROM posts p
LEFT JOIN comments c ON p.post_id = c.post_id
GROUP BY p.post_id;

-- DENORMALIZED:
ALTER TABLE posts ADD COLUMN comment_count INT DEFAULT 0;

-- Update via trigger
CREATE TRIGGER update_comment_count
AFTER INSERT ON comments
FOR EACH ROW
BEGIN
  UPDATE posts 
  SET comment_count = comment_count + 1 
  WHERE post_id = NEW.post_id;
END;

-- Now simple query (fast!):
SELECT * FROM posts WHERE post_id = 123;
-- No JOIN, no COUNT

-- Pattern 2: Duplicating lookup data
-- NORMALIZED:
CREATE TABLE orders (
  order_id INT PRIMARY KEY,
  product_id INT,
  quantity INT
);

CREATE TABLE products (
  product_id INT PRIMARY KEY,
  name VARCHAR(100),
  price DECIMAL(10,2)
);

-- DENORMALIZED (for historical accuracy):
CREATE TABLE orders_denorm (
  order_id INT PRIMARY KEY,
  product_id INT,
  product_name VARCHAR(100),   -- Snapshot!
  product_price DECIMAL(10,2), -- Snapshot!
  quantity INT
);
-- Hvis product price ændrer, påvirker det ikke historical orders

-- Pattern 3: Materialized path (tree structures)
-- NORMALIZED:
CREATE TABLE categories (
  id INT PRIMARY KEY,
  name VARCHAR(100),
  parent_id INT REFERENCES categories(id)
);

-- Find path kræver recursive query:
WITH RECURSIVE category_path AS (
  SELECT id, name, parent_id, name as path
  FROM categories WHERE id = 123
  UNION ALL
  SELECT c.id, c.name, c.parent_id, 
    cp.path || ' > ' || c.name
  FROM categories c
  JOIN category_path cp ON c.id = cp.parent_id
)
SELECT path FROM category_path;

-- DENORMALIZED:
ALTER TABLE categories ADD COLUMN path VARCHAR(500);
-- Electronics > Computers > Laptops

SELECT path FROM categories WHERE id = 123;
-- Instant!

-- NoSQL denormalization (MongoDB)
// NORMALIZED approach (anti-pattern i MongoDB):
// users collection:
{ _id: 1, name: "Peter", email: "peter@email.dk" }

// posts collection:
{ _id: 123, userId: 1, title: "Post Title" }

// Kræver $lookup (JOIN):
db.posts.aggregate([
  {
    $lookup: {
      from: "users",
      localField: "userId",
      foreignField: "_id",
      as: "author"
    }
  }
]);

// DENORMALIZED (embedded, MongoDB best practice):
{
  _id: 123,
  title: "Post Title",
  author: {
    id: 1,
    name: "Peter",  // Redundant men fast!
    email: "peter@email.dk"
  }
}

// Single query, no $lookup:
db.posts.findOne({ _id: 123 });

-- Maintaining denormalized data
-- Option 1: Application logic
await db.query('UPDATE users SET username = ? WHERE id = ?', 
  [newUsername, userId]);

// Update denormalized copies
await db.query(
  'UPDATE posts SET username = ? WHERE user_id = ?',
  [newUsername, userId]
);
await db.query(
  'UPDATE comments SET username = ? WHERE user_id = ?',
  [newUsername, userId]
);

-- Option 2: Database triggers (automatic)
CREATE TRIGGER sync_username
AFTER UPDATE ON users
FOR EACH ROW
WHEN (OLD.username != NEW.username)
BEGIN
  UPDATE posts SET username = NEW.username 
  WHERE user_id = NEW.user_id;
  
  UPDATE comments SET username = NEW.username
  WHERE user_id = NEW.user_id;
END;

-- Option 3: Batch sync job
// Run periodically
UPDATE posts p
SET username = (
  SELECT username FROM users WHERE id = p.user_id
)
WHERE username != (
  SELECT username FROM users WHERE id = p.user_id
);

-- Consistency risks
-- Problem: User updates username
UPDATE users SET username = 'NewName' WHERE id = 123;
-- Application crashes before updating posts!
-- Now posts have old username (inconsistent)

-- Solution: Use transactions
BEGIN TRANSACTION;
  UPDATE users SET username = 'NewName' WHERE id = 123;
  UPDATE posts SET username = 'NewName' WHERE user_id = 123;
COMMIT;

-- When to denormalize?
// 1. Read-heavy workloads (reads >> writes)
// 2. Complex JOINs impacting performance
// 3. Data that changes rarely (product names)
// 4. Acceptable eventual consistency

-- When NOT to denormalize?
// 1. Write-heavy workloads
// 2. Frequently changing data
// 3. Strict consistency requirements
// 4. Limited development resources (maintenance burden)