Update query-optimization.txt

[kerneljake]

The word "returned" implies that you can workaround this limitation using a projection, but in fact you cannot (per SERVER-2104).

[kay-kim]

Hi -- isn't the current page and the SERVER-2104 stating that if you have an index
db.foo.ensureIndex( { a: 1, "b.x": 1 } )
then
db.foo.find( { a: { $lt: 1 } }, { a: 1, _id: 0 } ) will be covered
whereas
db.foo.find( { a: { $lt: 1} }, { a: 1, "b.x": 1, _id: 0 } ) will not be covered

[kerneljake]

I agree with the above example, buy my example is different.
Below is an example of a literal interpretation of the sentence in an attempt to get around this limitation, but it doesn't work.

{code}

db.foo.insert({ tenant:1, user:{ name:"tester", password:"secret"}})

db.foo.ensureIndex({tenant:1, "user.name":1})

db.foo.find({tenant:1, "user.name":"tester"}, {user:0, _id:0}).explain(true).executionStats.totalDocsExamined
1
{code}

Since the indexed field in the subdocument is not returned, I would expect the query to be covered, but it is not covered. The phrase "any of the returned index fields are fields in subdocuments" implies that you can project away a subdocument containing an indexed field and still achieve a covered query.

[kay-kim]

ah -- I see. How about "If any of the indexed field in the query predicate or returned in the projection ..."
to cover both examples.

[kerneljake]

Yes, that sounds good!

[kay-kim]

merged with changes as discussed. thank you!