C100DBA Exam Dumps and Practice Questions

According to MongoDB aggregation pipeline optimization, when you have a $sort followed by a $match, the optimizer will automatically reorder these stages to move $match before $sort. This is called predicate pushdown or pipeline optimization. By filtering documents first with $match, you reduce the number of documents that need to be sorted, significantly improving performance. This optimization is done automatically by MongoDB's query optimizer, but it's best practice to write queries with $match early in the pipeline. Other optimizations include moving $match before $group, combining multiple $match stages, and eliminating unnecessary $project stages.

The $slice operator with the syntax {$slice: [skip, limit]} works as follows: the first argument (-10) specifies to start from the last 10 elements of the array. The second argument (5) specifies the limit - how many elements to return. So $slice: [-10, 5] returns the first 5 elements from the last 10 elements of the array. If the array had 20 comments, it would return comments at positions 10-14 (the first 5 of the last 10). A positive first argument would skip from the beginning, while a negative first argument skips from the end.

MongoDB compares BSON types in a specific order from lowest to highest priority. The complete type comparison order is: 1) Null, 2) Numbers (int, long, double), 3) Symbol and String, 4) Object, 5) Array, 6) Binary Data, 7) Object ID, 8) Boolean, 9) Date, 10) Timestamp, 11) Regular Expression. This ordering is important when performing comparison operations on fields that may contain different BSON types, as MongoDB will use this type ordering to determine which values are considered 'less than' or 'greater than' others when they are of different types.

When a secondary is prompted by another secondary for an election in a MongoDB replica set, the first task the secondary performs is to check its eligibility for election. It verifies whether it has the most recent oplog entries and meets the criteria to be a candidate (priority > 0, not hidden, not arbiter). If eligible, it increments its term and requests votes from other members. If not eligible, it rejects the request.

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.