MongoDB Pagination, Query Optimization, and Index Design Guide

MongoDB pagination can be implemented using the traditional skip() and limit() methods, but this becomes inefficient when skipping large offsets; instead, range‑based pagination using a sorted field (e.g., _id or a timestamp) retrieves the last document of the previous page and queries the next set with a $gt or $lt condition.

For descending order pagination, use

db.getCollection('file').find({_id:{ $lt: lastId }}).sort({_id:-1}).limit(pageSize)

; for ascending order, replace $lt with $gt and sort by _id:1. To obtain total record count, execute db.getCollection('file').find().count().

ObjectId values increase over time, but their granularity is seconds, so they may not guarantee ordering for inserts within the same second on distributed systems; a dedicated ordered field is preferable for reliable pagination.

MongoDB provides profiling commands to monitor slow queries: db.getProfilingLevel(), db.setProfilingLevel(level), and db.system.profile.find({millis:{$gt:500}}). The profiling output includes fields such as millis (query duration) and docsExamined (documents scanned), which help identify when indexes are missing.

When queries are slow, create indexes on the queried fields. Indexes use B‑Tree structures; they are beneficial for selective queries but can be counter‑productive for scans that retrieve a large portion of the collection.

Index design principles include: choosing primary keys wisely, preferring high‑cardinality fields, considering update frequency, avoiding prefix‑index limitations, adding redundant fields for long strings, preventing unnecessary indexes, aiming for query coverage, and limiting the number of indexed fields.

Use explain() to view the query execution plan, which shows stages such as COLLSCAN, IXSCAN, FETCH, SORT, LIMIT, and SKIP. Understanding these stages helps diagnose performance issues.

Additional considerations: if the dataset exceeds available RAM, query speed degrades; large indexes also consume memory; query order must match index order for optimal use; and replica sets can reduce write throughput proportionally to the number of members.