MongoDB for Developers with Python Transcripts
Chapter: High-performance MongoDB
Lecture: Concept: Projections
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One of the last simple tools you have in your tool belt when we're working with MongoEngine or even in PyMongo, just different api
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is this ability to restrict the data returned from the document. In our car object we've got the make, the model, the id, some other things,
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we've got the engine which is a subdocument or an embedded document there and then the biggest thing that contributes to the size
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is actually the service history which might be many service record entries. If really all we care about is the make, the model and the id of a car,
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and we're going to create like a list or something like that, we can use this .only operator here
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and dramatically reduce the amount of data returned from MongoDB so this is an operation that we saw when we first learned about the api
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actually operates at the database level, you're able to restrict the elements returned from the queries so when it gets back to MongoEngine
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basically it looks at what comes back and it says, alright, I need to create some cars and I need to set their make to this, the model to that
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and their id to whatever comes back, and then nothing else is transferred, deserialized, anything.
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So you can, if you don't need them, exclude the heavyweight things like the engine and the service histories for this particular use case.
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So this is kind of like select make, model, id from table such and such in SQL, and it really can improve the performance
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especially when you have either large documents or many documents. So you've seen a lot of different ways to turn the knobs of MongoDB
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to make it faster and to use MongoEngine to control those knobs. Now this applies to a single individual database server
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and if you use this to tune your database, you can actually make the need for having a sharded cluster and all these scaling things possibly go away,
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but even if you do end up with one of these more interesting topologies, all of these techniques still apply and they'll make your cluster go faster,
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they'll make your replicas go faster, all of those things. What you've learned here are really the foundational items of making MognoDB go fast.