05. Aggregation Pipeline Core: $match, $project, $group, $sort & $limit • MongoDB & NoSQL Architecture: CRUD to Sharding & WiredTiger • Rishabh Ki Kaksha

MongoDB & NoSQL Architecture: CRUD to Sharding & WiredTiger › 05. Aggregation Pipeline Core: $match, $project, $group, $sort & $limit

45 mins

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Course Syllabus

Module 1: MongoDB Foundations, BSON & CRUD Mechanics

01. NoSQL vs SQL: CAP Theorem & Document-Oriented Architecture 02. BSON Data Types & Document Schema Design (Embedding vs Referencing) 03. High-Performance CRUD Operations, BulkWrite & Write Concerns 04. Schema Validation with JSON Schema ($jsonSchema)

Module 2: Advanced Aggregation Framework & Data Pipelines

05. Aggregation Pipeline Core: $match, $project, $group, $sort & $limit 06. Complex Transformations: $unwind, $lookup (Joins) & $facet (Multi-Facet Analytics) 07. Window Operators ($setWindowFields) & Accumulator Functions

Module 3: Indexing Strategies & Query Optimization

08. B-Tree Indexes in MongoDB: Single Field, Compound & Partial Indexes 09. Special Indexes: TTL (Time-To-Live), Geospatial (2dsphere) & Text Indexes 10. Query Profiling, explain("executionStats") & Index Covering Analysis

Module 4: Storage Engine Internals (WiredTiger) & ACID Transactions

11. WiredTiger Storage Engine Architecture: Cache, Compression & Checkpoints 12. Read Concerns (local, majority, linearizable) & Write Concerns (w:majority, j:true) 13. Multi-Document Distributed ACID Transactions with Sessions

Module 5: High Availability, Replica Sets, Sharding & Backups

14. Replica Sets Architecture: Primary Election, Oplog & Automatic Failover 15. Sharding Architecture: mongos Router, Config Servers & Shard Key Selection 16. Range vs Hash Sharding, Chunk Balancing & Jumbo Chunks Mitigation 17. Change Streams: Building Real-Time Event-Driven Pipelines with MongoDB 18. Production Backup Strategies: mongodump/mongorestore, Point-in-Time & Atlas CLI

Interactive Lesson Studio

# 05. Aggregation Pipeline Core: $match, $project, $group, $sort & $limit

> MongoDB Notes by Rishabh - Topic 05

---

### 1. Aggregation Pipeline Kya Hai? (The Assembly Line Analogy 🏭)

Aggregation Pipeline ek sequential data transformation engine hai:
```
[ Raw Documents in Collection ]
              ⬇
  1. $match (Filter rows - uses Index!)
              ⬇
  2. $project (Reshape / Add computed fields)
              ⬇
  3. $group (Calculate sum, average, stats)
              ⬇
  4. $sort (Order the aggregated results)
              ⬇
  5. $limit (Return top 10 items)
              ⬇
[ Final Analytical Report ]
```

---

### 2. High-Performance E-Commerce Sales Aggregation

```javascript
await db.collection('orders').aggregate([
    // Stage 1: Filter only COMPLETED orders from 2026 (Uses Index!)
    {
        $match: {
            status: "COMPLETED",
            createdAt: { $gte: new Date("2026-01-01") }
        }
    },
    // Stage 2: Group by Customer ID & Calculate Total Spent + Order Count
    {
        $group: {
            _id: "$customerId",
            totalSpent: { $sum: "$totalAmount" },
            orderCount: { $sum: 1 },
            avgOrderValue: { $avg: "$totalAmount" }
        }
    },
    // Stage 3: Only keep High-Value VIP Customers (Spent > 50,000)
    {
        $match: {
            totalSpent: { $gt: 50000 }
        }
    },
    // Stage 4: Sort Descending by Total Revenue
    {
        $sort: { totalSpent: -1 }
    },
    // Stage 5: Top 10 VIPs
    {
        $limit: 10
    }
]).toArray();
```

---

### 3. The `$project` vs `$addFields` & `$unset` Modern Syntax

*   **`$project`:** White-list approach (Purani saari fields exclude ho jaati hain agar manually specify na karein).
*   **`$addFields`:** Existing document me naye computed fields jodta hai bina purani fields ko chhede!

---

### 4. Master Comparison Matrix

---

### 5. Interview Ke 3 Sawal Is Topic Se

1. **Aggregation Pipeline me 100MB RAM limit exceed hone par kya hota hai?** -> Query crash ho jaati hai with error; isko fix karne ke liye `{ allowDiskUse: true }` option pass karke temporary disk spill allow ki jaati hai.
2. **`$match` stage ko hamesha pipeline ke top par kyu rakhna chahiye?** -> Taaki B-Tree indexes leverage ho sakein aur aage ki memory stages me minimum documents pass hon (Early Filter Optimization).
3. **`$$ROOT` system variable aggregation me kya represent karta hai?** -> Current stage me process ho rahe poore root document ka reference.

---
**Next Topic:** 06. Complex Transformations: $unwind, $lookup (Joins) & $facet (Multi-Facet Analytics)

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