MONGODB · MEDIUM CODEX

MongoDB Interview Questions — Medium

Q: Explain the components of MongoDB aggregation framework ($match, $project, $group, $unwind, $lookup, $out).

$match: Filters documents to pass only matching documents to the next stage (equivalent to SQL WHERE). Must be placed as early as possible to utilize indexes. $project: Reshapes documents by adding, r

Q: What is the difference between `$merge` and `$out` stages in aggregation?

$out: Overwrites the entire output collection if it exists, or creates a new one. Cannot write to a sharded collection. Replaces the collection atomically upon pipeline completion. $merge: Introduces

Q: How does `$lookup` perform a join? What are its performance characteristics and index requirements?

Mechanism: Acts as a nested loop join. For each document in the source collection, it queries the target collection. Performance: Can be extremely slow if the target collection is large. Index Require

Q: What is the difference between single-field indexing and compound indexing regarding query coverage?

Single-Field Index: Indexes a single field (e.g., { a: 1 }). Optimizes queries filtering or sorting on a. Compound Index: Indexes multiple fields (e.g., { a: 1, b: -1 }). Optimizes queries filtering o

Q: Explain the performance implications of indexing array fields (multikey indexes).

Multikey Index: Automatically created when an index is created on a field containing an array. Under the Hood: MongoDB creates separate index keys for every single element in the array. Implications:

Real-world mechanisms, state management, edge cases, performance trade-offs, and practical coding.

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THEORY QUESTIONS & SOLUTIONSShowing 5 of 50 questions

Q1Explain the components of MongoDB aggregation framework ($match, $project, $group, $unwind, $lookup, $out).

$match: Filters documents to pass only matching documents to the next stage (equivalent to SQL WHERE). Must be placed as early as possible to utilize indexes.
$project: Reshapes documents by adding, renaming, or removing fields (equivalent to SQL SELECT).
$group: Groups documents by a specified key and performs accumulative operations (like $sum, $avg, $min, $max) on the grouped data.
$unwind: Deconstructs an array field from input documents to output a document for each element of the array.
$lookup: Performs a left outer join to another collection in the same database to filter/add data.
$out: Writes the resulting documents of the aggregation pipeline to a new or existing collection (must be the final stage).

Q2What is the difference between `$merge` and `$out` stages in aggregation?

$out:
- Overwrites the entire output collection if it exists, or creates a new one.
- Cannot write to a sharded collection.
- Replaces the collection atomically upon pipeline completion.
$merge:
- Introduces on-demand incremental updates. It can merge results (insert, update, discard, or fail on matches) into an existing target collection.
- Can write to sharded or unsharded collections, even across different databases.

Q3How does `$lookup` perform a join? What are its performance characteristics and index requirements?

Mechanism: Acts as a nested loop join. For each document in the source collection, it queries the target collection.
Performance: Can be extremely slow if the target collection is large.
Index Requirements: The joined foreign field on the target collection must be indexed. If not, MongoDB must perform a full collection scan on the target collection for every single document processed by $lookup.

Q4What is the difference between single-field indexing and compound indexing regarding query coverage?

Single-Field Index: Indexes a single field (e.g., { a: 1 }). Optimizes queries filtering or sorting on a.
Compound Index: Indexes multiple fields (e.g., { a: 1, b: -1 }). Optimizes queries filtering on a, or a and b.
Index Prefix Rule: A compound index { a, b } covers queries on a but not queries filtering only on b. To optimize queries on b, a separate single-field or compound index starting with b is required.

Q5Explain the performance implications of indexing array fields (multikey indexes).

Multikey Index: Automatically created when an index is created on a field containing an array.
Under the Hood: MongoDB creates separate index keys for every single element in the array.
Implications:
- High write overhead: If a document has an array with 100 elements, inserting it creates 100 index entries.
- Storage bloat: Multikey indexes occupy significantly more RAM and disk storage.
- Limit: A compound multikey index cannot have more than one array field indexed in the index definition to prevent exponential explosion of index entries.

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