When passing a $comment in a query count() doesn't use an index.
Testcase:
> for (var i=0; i < 1000000; i++) db.test.insert({a:i})
> db.test.count()
1000001
> db.test.count({$comment:"A"})
1000001
> var start = new ISODate(); db.test.count(); new ISODate() - start
0
> var start = new ISODate(); db.test.count({$comment:"A"}); new ISODate() - start
155
- is duplicated by
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SERVER-25830 $comment on an empty query causes a collection scan
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- Closed
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- is related to
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DOCS-9392 3.4: Behavior change for count with hint and empty query predicate on sparse index
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- Closed
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