concept size in category mongoDB

This is an excerpt from Manning's book MongoDB in Action, Second Edition: Covers MongoDB version 3.0. Login to get full access to this book.

Perhaps the most important thing to know about MongoDB’s use of memory-mapped files is how it affects data sets that exceed the size of available RAM. When you query such a data set, it often requires a disk access for data that has been swapped out of memory. The consequence is that many users report excellent MongoDB performance until the working set of their data exceeds memory and queries slow significantly. This problem isn’t exclusive to MongoDB, but it’s a common pitfall and something to watch.

Table 5.4. Summary of array operators

Operator	Description
$elemMatch	Matches if all supplied terms are in the same subdocument
$size	Matches if the size of the array subdocument is the same as the supplied literal value