Do Querries on large embedded Arrays kill performance?

[Marc Seeger]

Hi,
Take this example:
You try to build something like a filesystem index in mongodb:

[FOLDER] => [File1, File2, File3, ... FileN]

You have to check if a certain File is a already in a certain Folder
(only the name-strings, not the actual content!).
=> file_is_in_folder = !collection.find_one({:folder =>
folder_name, :files => file_name}).nil?

This seems to take a really long time for large (10.000+) collections
of filenames.
Is there any way that this could be speed up (aka: do indexes help on
embedded arrays).
If not: would putting the filenames as documents and setting an index
on foldername+filename help?

[Michael Dirolf]

You should index on the embedded array. See:
http://www.mongodb.org/display/DOCS/Multikeys

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[Marc Seeger]

If I only do queries for {:folder =>
folder_name, :files => file_name}, shouldn't I somehow make a compound
index?
Will making a compound index over the Folder and the Filenames-Array
automatically make a multikey on the embedded array?

[Michael Dirolf]

Ah sorry I missed that your queries looked like that. Yes you should
use a compound index - it will work w/ multikeys.