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The number of times the operation yielded to allow other operations to complete. Typically, operations yield when they need access to data that MongoDB has not yet fully read into memory. This allows other operations that have data in memory to complete while MongoDB reads in data for the yielding operation. For more information, see the FAQ on when operations yield.
Basically most database operations in MongoDB has a "yield point", i.e. the point that it can yield control to other operations. This is usually waiting for data to be loaded from disk.
So in short, if you see a high number of yields, that means the query spent a lot of time waiting for data to be loaded from disk. The cause is typically:
A query returning or processing a large amount of data
. If this is the cause, ensure the query only returns what you need. However this may not be avoidable in some use case (e.g. analytical).
Inefficient query that is not utilizing proper indexing, so the server was forced to load the full documents from disk all the time
. If this is the cause, ensure that you have created proper indexes backing the query.
Small RAM in the server, so the data must be loaded from disk all the time
. If none of the above, then the server is simply too small for the task at hand. Consider upgrading the server's hardware.
Note that a high number of yield is not necessarily bad if you don't see it all the time. However, it's certainly not good if you see this on a query that you run regularly.
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