In this new post to his site Kevin Schroeder has shared a helpful hint around the MongoDB library and excluding fields from the results of a query.

I am using the mongodb/mongodb library for a project of mine. The API seems fairly different from the old PECL library and it also came with some other, albeit unexpected, baggage. [...] One of the practices I’ve heard about Mongo is to get Mongo to do as much as it can, but not to worry too much about complicated joins and such as you would in SQL. In other words, don’t shy away from bringing data into the application to do some processing. That was the practice I followed, which worked fine up until my data size started to increase.

He started seeing some major performance issues when his data set grew to a significant size (50% of the response time). He went searching for a solution, tried MapReduce but eventually came upon an optional parameter letting him tell the Mongo DB to omit a value (or values) from the result set. Using this he dropped 7.5 seconds off of his wall clock time.