You have to understand that couchbase works differently to RDBMS's such as Oracle. Couchbase offers two ways for you to retrieve your data:
1) Key lookup, you know the key(s) of the document(s) that you want to retrieve.
2) Define Map Reduce jobs called Views which create indexes allowing you to query your data on attributes other than the key.
Couchbase documents are always consistent but views are not and are eventually consistent (although you have the ability to change this).
As the couchbase documentation states
Views are updated when the document data is persisted to disk. There is a delay between creating or updating the document, and the document being updated within the view.
So query time really depends on a variety of factors, can the view data be stale? How large is the data emitted from the index, and what is the current workload and db size? Couchbase provides the following 3 flags for working with views and how you want to access the data. False means the index has to be updated before returning the result, therefore it can potentially be slow.
- false : Force a view update before returning data
- ok : Allow stale views
- update_after : Allow stale view, update view after it has been accessed
Please check out the official document for more in depth answers http://docs.couchbase.com/couchbase-manual-2.2/#views-and-indexes
Also you can check out this interesting article on caching views http://blog.couchbase.com/caching-queries-couchbase-high-performance
Currently in development at Couchbase is N1QL, effectively the couchbase version of SQL, this will have the EXPLAIN statement available, this won't be released until late 2014 I believe.
A blog post introducing N1QL http://blog.couchbase.com/n1ql-it-makes-cents
A cheat sheet for N1QL http://www.couchbase.com/communities/sites/default/files/Couchbase-N1QL-CheatSheet.pdf
And where you can download the dev preview if you want to play with N1QL http://www.couchbase.com/communities/n1ql
Also checkout the cb stats tool http://docs.couchbase.com/couchbase-manual-2.2/#cbstats-tool it gives a high level overview of persistence rates,updates,key misses etc.