Errata

Hi, I guess this paragraph of chapter 5 is wrong.

The output of an explain on a query that uses an index varies, but in the simplest case it looks something like this if we add an index on imdb.rating:

Because we are not using any imdb database as example.

I hope you find this useful. Best regards.

The example code showing the use of "returnNewDocument" is missing "{".

db.processes.findOneAndUpdate(
...,
{"returnNewDocument": true})

Original text: However, if a document’s "x" field is an array, the document matches if there is an element of "x" that matches each part of the criteria, ...

Should be: However, if a document’s "x" field is an array, the document matches if there is a part of the criteria that matches each element of "x", ...

The text mentions two example indices, {"username": 1, "age": 1} and {"age": 1, "username": 1} . The indices used to generate the output in the following pages however, are {"username": 1} and {"age": 1, "username": 1}.

"30% or more" should be "30% or less".

An index is more effective if the returned resultset is a smaller fraction of the total amount of documents, so when the result set contains 30% or less of the collection the query is more likely to benefit from the index.

"username" should be "firstname"

The index in the output below is enforces a unique first name, not a unique username.

The coordinates for the polygon should be surrounded by an extra pair of square brackets. Furthermore, it is only valid GeoJson if the last coordinate in the polygon specification is the same as the first:
[[[0,1], [0,2], [1,2], [0,1]]]

Option --smallfiles is not supported in mongo 4.2

The comand to enable sharding on a single db should be :

sh.enableSharding("DB_NAME")

As reported : db.enableSharding("musci")

The commands for "archiving and compress the bcakup on a remote system using SSH", seemingly write the file on the wrong system. dd if=/dev/vg0/mdb-snap01 | ssh username@example.com gzip > /opt/backup/mdb-snap01.gz

This will send all the data from the volume group, to the gzip command on the remote host, however the redirection to file > is interpreted by the local file system, so the file will be written on the system calling ssh (local), and not gzip, so the data is copied back to the system.

Two lines later when you try and run gzip again the file will not be there.

I think, but didn't test, that the command should be:

dd if=/dev/vg0/mdb-sap01 | ssh username@examplec.com /bin/bash -c "gzip > opt/backup/mdb-snap01.gz"

You can run mongod.exe --help to see all the possible options.
--> You can run bin/mongod --help to see all the possible options.

"mongod.exe" should be "bin/mongod" if running on Linux/MacOS X.