Fix Answers in MongoDB assessment

[ed3899]

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• Missed questions/answers

Q.24 A compound index allows you to _ ?

• Use more than one field per index.

Reference: https://www.mongodb.com/docs/v6.0/core/index-compound/ Argument regarding the wrong answer: https://www.mongodb.com/docs/v6.0/reference/collation/#operations-that-support-collation

TLDR:

To use an index for string comparisons, an operation must also specify the same collation.

You cannot specify multiple collations for an operation. For example, you cannot specify different collations per field, or if performing a find with a sort, you cannot use one collation for the find and another for the sort.

For a compound index where the index prefix keys are not strings, arrays, and embedded documents, an operation that specifies a different collation can still use the index to support comparisons on the index prefix keys.

Q.45 What is NOT a standard role in MongoDB?

• delete collections

Reference: https://www.mongodb.com/docs/v6.0/reference/built-in-roles/#backup-and-restoration-roles TLDR: You won't find "delete collections" role in the document above

Q46. Which MongoDB shell command deletes a single document?

Answer is correct, just a note for future reference:

Note: db.collection.remove() is deprecated in the new mongosh. Use db.collection.deleteOne() or db.collection.deleteMany(). References:

• https://www.mongodb.com/docs/mongodb-shell/crud/delete/
• https://www.mongodb.com/docs/v6.0/reference/method/db.collection.remove/

Q49. You need to get the names of all the indexes on your current collection. What is the best way to accomplish this?

Correct answer, just a note for future reference:

Note: An alternative method in the mongosh shell is listIndexes()

Reference: https://www.mongodb.com/docs/v6.0/reference/command/listIndexes/#mongodb-dbcommand-dbcmd.listIndexes

Q50. You are going to do a series of updates to multiple records. You find setting the multi option of the update() command too tiresome. What should you do instead?

Correct answer, just a note for future reference:

Note: An alternative method for db is .update()

Reference: https://www.mongodb.com/docs/v6.0/reference/command/update/#mongodb-dbcommand-dbcmd.update

Q52. Given a customer collection which includes fields for gender and city, which aggregate pipeline shows the number of female customers in each city? (Alternative: How can you view the execution performance statistics for a query?)

Correct answer, just a note for future reference:

Note: If you want to analyze the performance of a query use .explain("executionStats") Reference: https://www.mongodb.com/docs/v6.0/tutorial/analyze-query-plan/

Q55. Which file in the MongoDB directly holds the MongoDB daemon?

• mongod Assuming that by file means executable. Docs

Q57. What is the most accurate statement regarding MongoDB and ad hoc queries?

• MongoDB allows ad hoc queries. Note: You don't need an index to perform ad hoc queries. Only pick one choice

Reference: https://www.mongodb.com/docs/v6.3/query-api/

Q.61 How can you improve the appearance of the output JSON that contains the _id?

This one is unclear.

1. db.collection.set() -> not a function
2. db.collection.format() -> not a function
3. Use $_id = value -> Assuming you do the following db.sales.find( { _id: ObjectId("64653c284df505e68d65733e") }) then it is possible to look for an item based on the id. But again...the output remains the same
4. Creating a second index, does nothing to the output...it only means faster read times but slower write times.

Q62. What happens to a Replica set oplog if it runs out of memory?

The oplog is capped collection and can't run out of memory

Argument:

Why "The oplog will be saved on one of the secondary servers." is wrong:

MongoDB applies database operations on the primary and then records the operations on the primary's oplog. The secondary members then copy and apply these operations in an asynchronous process. All replica set members contain a copy of the oplog, in the local.oplog.rs collection, which allows them to maintain the current state of the database.

Reasoning behind the right answer:

The oplog (operations log) is a special capped collection that keeps a rolling record of all operations that modify the data stored in your databases.

Unlike other capped collections, the oplog can grow past its configured size limit to avoid deleting the majority commit point.

Q63. MongoDB ships with a variety of files. Which file runs the MongoDB shell?

The answer is correct, just a note for future reference:

Note: mongosh is the new mongo shell, mongo is deprecated.

Starting in MongoDB v5.0, mongosh replaces mongo as the preferred shell.

Reference: https://www.mongodb.com/docs/mongodb-shell/

Q67. Suppose you are using the mongoimport command to import personnel data and there is a unique index on the email field. What happens when there are duplicate emails in the import?

May be two possible right answers here:

• The command doesn't prompt you to correct the bad error, but logs an error and aborts the operation.
• It adds everything up to and not including the duplicate record with:

E11000 duplicate key error collection: test.employees index: email_1 dup key: { email: "another1@example" }

I wasn't able to experiment with mongoimport because of software limitations. The error I got above was from doing inserts with the db.collections.insertMany api

Q69. You need to be able to quickly find a word in a text field. What should you do?

• Create a text index on the field and do a $text Query.

Argument: You need a text index in order to perform a $text query on a field. $text query uses the text index under the hood References:

• $text query
• Text index

Q71. After installing MongoDB on your machine, what must you do before launching Mongo?

• Create a data directory.

Note: The question in case is ambiguous. In the mongo docs, on the Windows Installation section, it clearly specifies the need for creating a data directory. However, that does not seem to be the case for Unix based systems. So, that gives use the closest possible solution for a specific platform. If we extrapolate that to the question, that seems to be the most reasonable solution.

Q79. You need to add an index to the large name collection in your production database. You do not want to have disruption of service for your users and you can't afford to have a team to do the work during after hours. What should you do?

Answer is correct, just a note for future reference:

Concurrency Changed in version 4.2.

MongoDB uses an optimized build process that obtains and holds an exclusive lock on the specified collection at the start and end of the index build. All subsequent operations on the collection must wait until createIndex() releases the exclusive lock. createIndex() allows interleaving read and write operations during the majority of the index build.

For more information on the locking behavior of createIndex() , see Index Builds on Populated Collections.

Reference: https://www.mongodb.com/docs/v6.0/reference/method/db.collection.find/#mongodb-method-db.collection.find

Q82. When using the mongoimport command, how can you drop the database before importing?

• Drop the database manually before importing.

Argument: There is no -d option in the docshttps://www.mongodb.com/docs/database-tools/mongoimport/#options.

Note: Assuming you are asked to drop a collection instead while importing, the use the --drop option.

[arpitajana1220]

I would like to work on this issue. Could you please assign this issue to me? @ed3899 @Ebazhanov

[arpitajana1220]

I have fixed this in : 1e8a6b07d8544a1149459856727a76c8f8b9e3a0