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Suppose that after $match stage, 10,000 documents will be passed into a $lookup stage. For each of those 10,000 documents, the $lookup will need to join based on its _id. The foreign field to join by will be indexed and almost unique - as it is made up of _id.

Will each $lookup for each of the 10,000 documents have O(1) due to the index?

So is it safe to assume that this pipeline will be fast and scalable?

Bear Bile Farming is Torture
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  • Never assume, test and verify. – Joe Jan 26 '23 at 07:12
  • @Joe But this question seems to be answerable based upon raw first principles, no? – Bear Bile Farming is Torture Jan 27 '23 at 16:51
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    If all that matters is big O, sure, but at some point the amount of memory, disk bandwidth, connection counts, etc. have a bigger impact on scalability. So big O is a great tool for getting started, but it would be foolhardy to say that it is safe to assume scalability based solely on big O. – Joe Jan 27 '23 at 21:02

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