파이프라인에서 $sort, $limit, $skip 사용하기

// Sort by revenue descending, then alphabetically by name for ties
db.products.aggregate([
  { $group: { _id: '$category', revenue: { $sum: '$price' } } },
  { $sort: { revenue: -1, _id: 1 } }  // revenue desc, category name asc
]);

// Index-backed sort: $sort on indexed field before any transformation
db.orders.aggregate([
  { $match: { userId: 'u1' } },
  { $sort: { createdAt: -1 } }  // if createdAt is indexed, no in-memory sort
]);

// In-memory sort: $sort after $group (computed field, no index)
db.orders.aggregate([
  { $group: { _id: '$userId', total: { $sum: '$amount' } } },
  { $sort: { total: -1 } }  // in-memory: 'total' is a computed field
]);

// Top 10 best-selling products
db.orders.aggregate([
  { $group: { _id: '$productId', sold: { $sum: '$qty' } } },
  { $sort: { sold: -1 } },
  { $limit: 10 },  // only top 10 pass through
  // Only 10 lookups instead of thousands:
  { $lookup: { from: 'products', localField: '_id', foreignField: '_id', as: 'product' } }
]);

const PAGE = 2;
const PAGE_SIZE = 10;

db.articles.aggregate([
  { $match: { status: 'published' } },
  { $sort: { publishedAt: -1 } },
  { $skip: (PAGE - 1) * PAGE_SIZE },  // skip page 1's 10 docs
  { $limit: PAGE_SIZE }               // take page 2's 10 docs
]);

// This pattern triggers the top-N sort optimisation
db.reviews.aggregate([
  { $match: { productId: 'p1' } },
  { $sort: { rating: -1, helpful: -1 } },  // sort
  { $limit: 5 }   // $sort + $limit merged into top-5 sort internally
]);
// MongoDB never sorts ALL reviews; it tracks only the best 5

// You write:
db.orders.aggregate([
  { $sort: { amount: -1 } },
  { $match: { status: 'completed' } }  // optimizer moves this BEFORE $sort
]);

// Optimized execution (equivalent to):
db.orders.aggregate([
  { $match: { status: 'completed' } },  // fewer docs to sort
  { $sort: { amount: -1 } }
]);

async function getProducts({ category, sort = 'price', page = 1, pageSize = 20 }) {
  return db.products.aggregate([
    { $match: { category, inStock: true } },   // 1. filter
    { $sort: { [sort]: 1 } },                  // 2. sort
    { $skip: (page - 1) * pageSize },          // 3. offset
    { $limit: pageSize }                       // 4. take page
  ]).toArray();
}

// Page 1000 is SLOW: MongoDB processes and discards 19,980 docs
db.posts.aggregate([
  { $sort: { createdAt: -1 } },
  { $skip: 19980 },  // 999 * 20
  { $limit: 20 }
]);

// Keyset pagination: always fast regardless of page depth
db.posts.aggregate([
  { $match: { createdAt: { $lt: lastSeenTimestamp } } },  // cursor
  { $sort: { createdAt: -1 } },
  { $limit: 20 }
]);

// Enable disk use for large sorts
db.events.aggregate(
  [
    { $match: { year: 2024 } },
    { $sort: { timestamp: -1 } },
    { $group: { _id: '$userId', events: { $push: '$type' } } }
  ],
  { allowDiskUse: true }  // allows spilling to disk
);

db.products.aggregate([
  { $match: { category: 'electronics' } },
  { $sort: { price: 1 } },
  { $facet: {
    // Branch 1: paginated results
    data: [
      { $skip: 0 },
      { $limit: 20 }
    ],
    // Branch 2: total count
    total: [
      { $count: 'count' }
    ]
  }}
]);
// Output: { data: [...20 docs...], total: [{ count: 348 }] }