How efficient is the MongoDB $in operator?

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0

Let's say I will have millions of comments that are cross-posted to multiple points of interest. Should I store each cross-posted comment multiple time:

{
  username: ..., 
  text: ...,
  poi_id: some_id1
}
{
  username: ..., 
  text: ...,
  poi_id: some_id2
}

or should I do it as an array

{
  username: ..., 
  text: ...,
  poi_ids: [some_id1, some_id2]
}

The first can just use an index on poi_id and be blazingly fast. The second one makes more intuitive sense as a database structure, and saves space, but I'm worried about query efficiency of using $in if I want to, say, get all the comments associated with some_id1.

asked 54 secs ago

wuxiekeji

52

How efficient is the MongoDB $in operator?