Firebase Performance: How many children per node?

Question

If a node has 100 million children, will there be a performance impact if I:

a) Query, but limit to 10 results

b) Watch one of the children only

I could split the data up into multiple parents, but in my case I will have a reference to the child so can directly look it up (which reduces the complexity). If there is an impact, what is the maximum number for each scenario before performance is degraded?

Answer 1

If a node has that many children, accessing the node in any way is a recipe for problems. Accessing an individual child is never a problem.

Querying the node for a subset of its children still requires that the database consider each of those children. If you request the last 10 out of 100 million items, you're asking the database to consider 999,999,990 items that you're apparently not interested in.

It is impossible to say what the maximum is without a way more concrete description of the data size, ordering criteria, etc. But to be honest, even then the best you're likely to get is a value with a huge variance that is likely to change over time.

You best approach in Firebase (and most NoSQL solutions) is to model the data in a way that fits with how your app uses that data. So for example: if you need to show the latest 10 items to your users, store the (keys of) those latest 10 items in a separate list.

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I'm not sure if I got the right number of nines in there, but I hope you get the idea.