面试题与答案

MongoDB sacrifices ACID properties for performance and scalability. However, it supports multi-document transactions in recent versions to some extent.

The `explain` method provides information on the query execution plan, helping in optimizing and understanding how a query is executed.

Covered queries are queries where all the fields in the query are part of an index, and the index itself can fulfill the query requirements, avoiding the need to fetch data from the actual documents.

The `$lookup` stage is used in MongoDB aggregation to perform a left outer join between documents from the source and target collections based on a specified condition.

The `mongostat` command provides a real-time view of various MongoDB statistics, such as insert and query rates, memory usage, and more.

MongoDB supports multi-document transactions starting from version 4.0. Transactions allow you to perform multiple operations on one or more documents and ensure atomicity and consistency.

The Aggregation Pipeline is a framework in MongoDB for data aggregation. It allows you to process data and transform it using a sequence of stages, such as `$match`, `$group`, and `$project`.