indexing - Clarification on sorting subsets in mongodb -


i have documents fields a, b, c, d, e, p in mongodb. have indexes { a: 1 }, { b: 1 },{ c: 1 }, { d: 1 },{ e: 1 }, { p: 1 }, compound indexes { a: 1, b: 1, c: 1 }. field p represents position of fields.

if have select this: { a: 'something', b: 'something else', c: 'and again' }, know use index { a: 1, b: 1, c: 1 } search (obviously).

however, if want sort p ( { p: 1 }), indexes used sort data? worried because can potentially have large dataset.

i read http://docs.mongodb.org/manual/tutorial/sort-results-with-indexes/ , cannot quite figure out use case (although it's there)

probably best said "can mongodb use 1 index match , 1 sort?" , answer no.

the thing "looking" in explain output see if index used sort value scanandorder show true when index cannot used , false can.

considering adapted sample, compound index on "a", "b", , "c" , separate index on "p", query this:

db.collection.find( { a: "a", b: "b" }).sort({ p: 1 }).explain()

would show compound index used query, not used sort. if had index created this:

db.collection.ensureindex({ a: 1, b: 1, p: 1 })

then index would used sort. in similar fashion, if has created this:

db.collection.ensureindex({ p: 1, a: 1, b: 1 })

then index selected , done sort action , used sort. not efficient of values a , b scanned in index determine result.

as final case small anount of documents , original compound selections optimizer pick "p" index, given small amount of documents scan fact use index affect sort operation.


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