Is mongoDB efficient in doing multi-key lookups?

Question

I'm evaluating MongoDB, coming from Membased/memcached because I want more flexibility.

Of course Membase is excellent in doing fast (multi)-key lookups.

I like the additional options that MongoDB gives me, but is it also fast in doing multi-key lookups? I've seen the $or and $in operator and I'm sure I can model it with that. I just want to know if it's performant (in the same league) as Membase.

use-case, e.g., Lucene/Solr returns 20 product-ids. Lookup these product-ids in Couchdb to return docs/ appropriate fields.

Thanks, Geert-Jan

Answer 1

For your use case, I'd say it is, from my experience: I hacked some analytics into a database of mine that made a lot of $in queries with thousands of ids and it worked fine (it was a hack). To my surprise, it worked rather well, in the lower millisecond area.

Of course, it's hard to compare this, and -as usual- theory is a bad companion when it comes to performance. I guess the best way to figure it out is to migrate some test data and send some queries to the system.

Use MongoDB's excellent built-in profiler, use $explain, keep the one index per query rule in mind, take a look at the logs, keep an eye on mongostat, and do some benchmarks. This shouldn't take too long and give you a definite and affirmative answer. If your queries turn out slow, people here and on the news group probably have some ideas how to improve the exact query, or the indexation.

Answer 2

One index per query. It's sometimes thought that queries on multiple keys can use multiple indexes; this is not the case with MongoDB. If you have a query that selects on multiple keys, and you want that query to use an index efficiently, then a compound-key index is necessary.

http://www.mongodb.org/display/DOCS/Indexing+Advice+and+FAQ#IndexingAdviceandFAQ-Oneindexperquery.

There's more information on that page as well with regard to Indexes.

The bottom line is Mongo will be great if your indexes are in memory and you are indexing on the columns you want to query using composite keys. If you have poor indexing then your performance will suffer as a result. This is pretty much in line with most systems.