Question

I’ve got a mongoDB collection that I’ve created via PHP. Now I want to index several fields, however the php documentation doesn’t talk about how to not apply a sort to the index!

For example, I want to index field1, field2, field3, and have the whole thing sorted by field3.

Since I will be searching the collection based on field1, field2, I wanted to make sure those have indexes, but when the collection returns from a find() operation, I just wanted it to be sorted by field3.

Any thoughts?

Answer 1

I thought it would be useful for me to give you a bit of background information before fully answering your question:

When mongo creates an index on a collection it is essentially sorting the collection based on the index key. Thus, when you get the results from a query that uses an index, the results are going to be sorted by key for that index. If your query uses a basic index on field1, then the results are, by definition, sorted on that index.

There are a few ways to get around this however. One simple way would be to

1) Query on field1

2) Sort the result of 1) using field3

This would, if I understand your comment correctly, get you your desired output. However, it has the downside of requiring 2 major pieces of work, which could be costly depending on the size of your data.

There is another way to solve your problem: compound indexes. Compound indexes use multiple keys to generate an index that is sorted based on both of those keys. There is some documentation here: http://www.mongodb.org/display/DOCS/Indexes#Indexes-CompoundKeys and there are quite a few stack-overflow questions about compound indexes if you want to dive more deeply into the subject.

Also, if you want some general info on indexing (and compound indexing) in mongodb check out http://www.mongodb.org/display/DOCS/Indexing+Advice+and+FAQ#IndexingAdviceandFAQ-1.Thesortcolumnmustbethelastcolumnusedintheindex. this whole document should be really useful to you I expect.

Using compound indexes, while elegant and appropriate, is definitely more complex than just using sort after a query. If you are willing to spend the time looking into it though I am sure you would find it will help you find a nice solution.

Hope this helps!