Difference between MongoDB's find and findone calls
Based on my own benchmarks, find().limit(1) is orders of magnitude faster than findOne().

There is either an error in the MongoDB documentation or a bug in findOne(). findOne() performs more like find().limit(N) where N is the number of documents the query would return. I figured this out while trying to figure out why my simple queries were so slow!

update: response from a 10gen (MongoDB) engineer:


  The two queries you are executing are very different. A find query
  returns a cursor, this is essentially a no-operation scenario, as no
  actual data is returned (only the cursor information). If you call
  findOne, then you are actually returning the data and closing the
  cursor. The docs should definitely be clearer :-)


Update: Indeed, if the find().limit(1) document is retrieved, the orders of magnitude speed difference seems to disappear. Also, I could not reproduce the major speed difference with the MongoDB JavaScript driver. I originally benchmarked using the MongoDB Java driver.