In situation where there are two plans that tie, if one matches more (leading?) keys in the query than another then it's more likely to be a better plan for other queries of this shape.
Example:
db.collection.find({a:999, b:111, c:"x"}).sort({d:-1})
// indexes for collection:
{a:1,b:1,c:1,d:1}
{a:1,b:1,d:1}
{a:1,d:1}
For value of a in query that matches one or no documents all three indexes are going to perform equally (yes?) - we should break the tie in favor of the first one because it matches the most predicate fields in the query.
- duplicates
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SERVER-80233 Implement index prefix heuristic
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- Closed
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- is duplicated by
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SERVER-59337 Bad index selection when using $in
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- Closed
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SERVER-40844 Better tie breaking of "perfect" indexes
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- Closed
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- is fixed by
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- Closed
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- related to
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- Closed
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