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Which of the following commands can cause the database to be locked?
Database locking can occur with various operations in MongoDB. Operations that can cause database-level or collection-level locks include: administrative operations like drop, certain index operations, compact operations, and other maintenance tasks. However, in modern MongoDB versions with document-level locking, most CRUD operations do not lock the entire database. Specific operations that may lock include administrative operations and certain maintenance tasks.
Which of the following collections stores authentication credentials in MongoDB?
What does the following aggregate query perform?
Without seeing the specific aggregate query and available options, I cannot provide a definitive answer. However, a general explanation: Aggregate queries in MongoDB use the aggregation pipeline to process data through multiple stages (match, group, project, sort, etc.). To answer this question, you would need to analyze what stages are used in the query and what transformations they perform on the data.
Consider the following document:
> db.c.find()
{ "_id" : 12, b : [ 3, 5, 7, 2, 1, -4, 3, 12 ] }
Which of the following queries on the "c" collection will return only the first five elements of the array in the "b"
field? E.g.,
Document you want returned by your query:
{ "_id" : 12, "b" : [ 3, 5, 7, 2, 1 ] >
To return only the first five elements of an array field in MongoDB, use the $slice projection operator.
The correct query would be:
db.c.find({}, {b: {$slice: 5}})Or more explicitly:
db.c.find({}, {b: {$slice: [0, 5]}})How it works:
$slice: 5 - Returns the first 5 elements of the array$slice: [0, 5] - Returns 5 elements starting from index 0$slice: -5 - Would return the last 5 elements$slice: [2, 3] - Would skip first 2 elements and return next 3Without seeing the specific options provided in the question, ensure the correct answer uses the $slice projection operator with a value of 5 or [0, 5].
Given a collection posts as shown below having a document array comments, which of the following command will create an index on the comment author descending?
To create an index on a nested field within an array (like the author field within the comments array), use dot notation. The syntax is db.posts.createIndex({'comments.author': -1}), where -1 indicates descending order. This creates a multikey index automatically since comments is an array field.