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I'm trying to find out where there is a discussion about using a document for an _id value.
Specifically, one that lists the gotchas of exact matches requiring equivalent field order, and when the default _id index is and is not used (
{ { _id: { a: 1, b: 2 } }
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vs.
{ "_id.a": 1, "_id.b": 2 }
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).
We just solved a production performance issue by adding an index on effectively "_id.a" and so I'm trying to find a nice link to add to our required-knowledge list that spells that out. So far the best I have found is:
http://stackoverflow.com/questions/23164417/mongodb-and-composite-primary-keys#answer-23317292
But it doesn't list the reasons why, just the facts.
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