[dkhenry]

What should be added to this is the ability to tell Mongo to return an error if your query requires a table scan. That one configuration item can be the difference between a request returning an error and a whole site going down for an extended period.

[pbrumm]

There is a config setting for that

  notablescan = true

http://docs.mongodb.org/manual/reference/configuration-optio...

[dkhenry]

Right I was saying that the author should add that to his article. Its a very valuable configuration item and anytime your dealing with optimizing indexes its worth enabling.

[lucian1900]

You can ask it to explain what it did, just like you'd ask for the last error. Then in your app enforce whatever rules related to indexes you wish.

[mattparlane]

explain actually runs the query. From http://www.mongodb.org/display/DOCS/Explain :

    Note that explain runs the actual query to determine the result. If the query is slow, the explain will be slow too.