-- B-tree index (default in PostgreSQL/MySQL)
CREATE INDEX idx_orders_user_date ON orders (user_id, created_at DESC);

-- Covering index: includes all columns query needs
CREATE INDEX idx_users_email_cover ON users (email)
  INCLUDE (name, status);    -- PostgreSQL syntax; avoids heap fetch

-- Partial index: only active users (sparse, smaller)
CREATE INDEX idx_active_users ON users (email) WHERE status = 'active';

-- Expression index
CREATE INDEX idx_lower_email ON users (LOWER(email));
-- Enables: WHERE LOWER(email) = 'alice@example.com'

-- EXPLAIN ANALYZE to see index usage
EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)
SELECT * FROM orders WHERE user_id = 42 AND created_at > NOW() - INTERVAL '30 days';

-- Look for: Index Scan vs Seq Scan, cost estimates, actual rows

-- Sample: employees(id, name, dept, salary, hire_date)

-- ── Ranking functions ─────────────────────────────────────
SELECT
  name, dept, salary,
  ROW_NUMBER() OVER (PARTITION BY dept ORDER BY salary DESC) AS row_num,
  -- No gaps, no ties: 1,2,3,4
  RANK()       OVER (PARTITION BY dept ORDER BY salary DESC) AS rank,
  -- Gaps for ties: 1,2,2,4
  DENSE_RANK() OVER (PARTITION BY dept ORDER BY salary DESC) AS dense_rank,
  -- No gaps: 1,2,2,3
  NTILE(4)     OVER (ORDER BY salary) AS quartile
  -- Divides into n equal buckets
FROM employees;

-- ── Lead / Lag — access adjacent rows ────────────────────
SELECT
  name, hire_date, salary,
  LAG(salary,  1, 0) OVER (PARTITION BY dept ORDER BY hire_date) AS prev_salary,
  LEAD(salary, 1, 0) OVER (PARTITION BY dept ORDER BY hire_date) AS next_salary,
  salary - LAG(salary, 1) OVER (ORDER BY hire_date) AS salary_change
FROM employees;

-- ── Running totals and moving averages ────────────────────
SELECT
  name, dept, salary, hire_date,
  SUM(salary)  OVER (PARTITION BY dept ORDER BY hire_date
                     ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS running_total,
  AVG(salary)  OVER (PARTITION BY dept ORDER BY hire_date
                     ROWS BETWEEN 2 PRECEDING AND CURRENT ROW) AS moving_avg_3,
  COUNT(*)     OVER (PARTITION BY dept) AS dept_headcount,
  MAX(salary)  OVER (PARTITION BY dept) AS dept_max_salary,
  salary / SUM(salary) OVER (PARTITION BY dept) * 100 AS pct_of_dept_payroll
FROM employees;

-- ── Top-N per group (classic interview problem) ───────────
SELECT * FROM (
  SELECT *, ROW_NUMBER() OVER (PARTITION BY dept ORDER BY salary DESC) AS rn
  FROM employees
) ranked
WHERE rn <= 3;   -- top 3 earners per department

-- ── First/Last value ──────────────────────────────────────
SELECT
  name, dept, salary,
  FIRST_VALUE(name) OVER (PARTITION BY dept ORDER BY salary DESC) AS top_earner,
  LAST_VALUE(salary) OVER (
    PARTITION BY dept ORDER BY salary DESC
    ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
  ) AS lowest_in_dept
FROM employees;

-- Find users with no orders (LEFT JOIN + IS NULL pattern)
SELECT u.id, u.name
FROM users u
LEFT JOIN orders o ON u.id = o.user_id
WHERE o.id IS NULL;
-- Alternative: NOT EXISTS (often faster with proper index)
SELECT id, name FROM users u
WHERE NOT EXISTS (SELECT 1 FROM orders WHERE user_id = u.id);

-- Self join: employee + manager
SELECT e.name AS employee, m.name AS manager
FROM employees e
LEFT JOIN employees m ON e.manager_id = m.id;  -- LEFT to include top-level (CEO)

-- Multiple joins
SELECT o.id, u.name, p.name AS product, o.quantity
FROM orders o
JOIN users    u ON o.user_id    = u.id
JOIN products p ON o.product_id = p.id
WHERE o.created_at >= CURRENT_DATE - INTERVAL '7 days';

-- Set isolation level
BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
  -- critical financial operation
  UPDATE accounts SET balance = balance - 500 WHERE id = 1;
  UPDATE accounts SET balance = balance + 500 WHERE id = 2;
COMMIT;

-- Optimistic locking (avoid long-held locks)
BEGIN;
  SELECT version FROM inventory WHERE product_id = 42;  -- version=5
  -- ... do business logic ...
  UPDATE inventory SET qty = qty - 1, version = version + 1
  WHERE product_id = 42 AND version = 5;  -- fails if version changed
  -- check affected rows; if 0, retry
COMMIT;

-- ── Basic CTE ─────────────────────────────────────────────
WITH dept_avg AS (
  SELECT dept_id, AVG(salary) AS avg_salary
  FROM employees
  GROUP BY dept_id
),
high_earners AS (
  SELECT e.*, d.avg_salary,
         e.salary - d.avg_salary AS above_avg
  FROM employees e
  JOIN dept_avg d ON e.dept_id = d.dept_id
  WHERE e.salary > d.avg_salary
)
SELECT name, dept_id, salary, above_avg
FROM high_earners
ORDER BY above_avg DESC;

-- ── Recursive CTE — org chart (manager hierarchy) ─────────
WITH RECURSIVE org_tree AS (
  -- Base case: top-level managers (no manager)
  SELECT id, name, manager_id, 1 AS level, CAST(name AS VARCHAR(1000)) AS path
  FROM employees
  WHERE manager_id IS NULL

  UNION ALL

  -- Recursive case: employees with a manager
  SELECT e.id, e.name, e.manager_id, ot.level + 1,
         CAST(ot.path || ' > ' || e.name AS VARCHAR(1000))
  FROM employees e
  JOIN org_tree ot ON e.manager_id = ot.id
)
SELECT id, name, level, path
FROM org_tree
ORDER BY path;

-- ── Recursive CTE — find all paths in a graph ──────────────
WITH RECURSIVE paths AS (
  SELECT from_node, to_node, ARRAY[from_node, to_node] AS path
  FROM edges
  WHERE from_node = 1    -- start node

  UNION ALL

  SELECT p.from_node, e.to_node, p.path || e.to_node
  FROM paths p
  JOIN edges e ON p.to_node = e.from_node
  WHERE NOT (e.to_node = ANY(p.path))   -- avoid cycles
)
SELECT * FROM paths WHERE to_node = 10;  -- find all paths to node 10

from pymongo import MongoClient, ASCENDING, DESCENDING
from datetime import datetime, timedelta

client = MongoClient("mongodb://localhost:27017")
db     = client["ecommerce"]
orders = db["orders"]

# ── CRUD ─────────────────────────────────────────────────
# Insert
doc_id = orders.insert_one({
    "user_id": "u123",
    "items": [
        {"product": "Laptop", "qty": 1, "price": 999.99},
        {"product": "Mouse",  "qty": 2, "price": 29.99},
    ],
    "status": "pending",
    "created_at": datetime.utcnow(),
    "total": 1059.97
}).inserted_id

# Query
recent = list(orders.find(
    {"status": "shipped", "created_at": {"$gte": datetime.utcnow() - timedelta(days=7)}},
    {"user_id": 1, "total": 1, "_id": 0}  # projection
).sort("created_at", DESCENDING).limit(20))

# Update — atomic operators
orders.update_one(
    {"_id": doc_id},
    {
        "$set":  {"status": "shipped"},
        "$push": {"tracking": {"event": "dispatched", "ts": datetime.utcnow()}},
        "$inc":  {"update_count": 1}
    }
)

# ── Aggregation Pipeline ──────────────────────────────────
pipeline = [
    # Stage 1: Filter
    {"$match": {
        "created_at": {"$gte": datetime(2024, 1, 1)},
        "status": {"$in": ["shipped", "delivered"]}
    }},
    # Stage 2: Unwind array
    {"$unwind": "$items"},
    # Stage 3: Group
    {"$group": {
        "_id": "$items.product",
        "total_revenue": {"$sum": {"$multiply": ["$items.qty", "$items.price"]}},
        "units_sold":    {"$sum": "$items.qty"},
        "order_count":   {"$addToSet": "$_id"}
    }},
    # Stage 4: Add computed field
    {"$addFields": {
        "unique_orders": {"$size": "$order_count"},
        "avg_price": {"$divide": ["$total_revenue", "$units_sold"]}
    }},
    # Stage 5: Sort
    {"$sort": {"total_revenue": -1}},
    # Stage 6: Limit
    {"$limit": 10},
    # Stage 7: Project output
    {"$project": {
        "product": "$_id", "_id": 0,
        "total_revenue": {"$round": ["$total_revenue", 2]},
        "units_sold": 1, "unique_orders": 1
    }}
]
results = list(orders.aggregate(pipeline))

# ── Indexes ────────────────────────────────────────────────
orders.create_index([("user_id", ASCENDING), ("created_at", DESCENDING)])
orders.create_index([("status", ASCENDING)], partial_filter_expression={"status": {"$ne": "deleted"}})
orders.create_index([("items.product", "text")])  # full-text index

-- CQL (Cassandra Query Language)

-- Schema design: query-first approach
-- Q: "Get all events for a user, newest first"
CREATE TABLE user_events (
  user_id   UUID,
  event_ts  TIMESTAMP,
  event_id  UUID,
  type      TEXT,
  payload   MAP,
  PRIMARY KEY (user_id, event_ts, event_id)
) WITH CLUSTERING ORDER BY (event_ts DESC, event_id ASC)
  AND default_time_to_live = 7776000;  -- auto-expire after 90 days

-- Partition key = user_id (all events for a user on one node)
-- Clustering = event_ts DESC (newest first without sorting)

-- Q: "Get events for user in last 24 hours"
SELECT * FROM user_events
WHERE user_id = f47ac10b-...
  AND event_ts >= toTimestamp(now()) - 86400s
LIMIT 100;

-- Tunable consistency (per-query)
-- Write with QUORUM (majority of replicas)
INSERT INTO user_events ... USING CONSISTENCY QUORUM;

-- Read with ONE (fastest, eventual consistency)
SELECT ... FROM user_events WHERE ... CONSISTENCY ONE;

-- QUORUM for both = strong consistency (safe but slower)
-- Replication factor 3: QUORUM = 2 replicas must ack

import redis, time, json

r = redis.Redis(host='localhost', port=6379, decode_responses=True)

# ── Caching with TTL ───────────────────────────────────────
def get_user(user_id: str) -> dict:
    key = f"user:{user_id}"
    cached = r.get(key)
    if cached:
        return json.loads(cached)
    user = db.query_user(user_id)           # slow DB query
    r.setex(key, 3600, json.dumps(user))    # cache 1 hour
    return user

# ── Distributed Rate Limiting (token bucket) ──────────────
def rate_limit(user_id: str, limit: int = 100, window: int = 60) -> bool:
    key = f"rate:{user_id}"
    pipe = r.pipeline()
    now  = time.time()
    pipe.zremrangebyscore(key, 0, now - window)    # remove old requests
    pipe.zadd(key, {str(now): now})                # add current request
    pipe.zcard(key)                                # count in window
    pipe.expire(key, window)
    _, _, count, _ = pipe.execute()
    return count <= limit

# ── Distributed Lock (SETNX pattern) ──────────────────────
import uuid
def acquire_lock(resource: str, ttl: int = 10) -> str | None:
    lock_id = str(uuid.uuid4())
    acquired = r.set(f"lock:{resource}", lock_id, nx=True, ex=ttl)
    return lock_id if acquired else None

def release_lock(resource: str, lock_id: str) -> bool:
    # Lua script for atomic check-and-delete
    lua = """
    if redis.call("get", KEYS[1]) == ARGV[1] then
        return redis.call("del", KEYS[1])
    else return 0 end"""
    return bool(r.eval(lua, 1, f"lock:{resource}", lock_id))

# ── Leaderboard (Sorted Set) ───────────────────────────────
r.zadd("leaderboard:2024", {"alice": 9500, "bob": 8700, "carol": 9200})
# Top 10
r.zrevrange("leaderboard:2024", 0, 9, withscores=True)
# User rank
r.zrevrank("leaderboard:2024", "alice")  # 0-indexed rank

# ── Pub/Sub ────────────────────────────────────────────────
# Publisher
r.publish("notifications:user:u123", json.dumps({"type": "order_shipped", "order_id": "o456"}))
# Subscriber (blocking)
pubsub = r.pubsub()
pubsub.subscribe("notifications:user:u123")
for message in pubsub.listen():
    if message["type"] == "message":
        print(json.loads(message["data"]))