mongodb compound index over extending - mongodb

Question

I have a question regarding compound indexes that i cant seem to find, or maybe just have misunderstood.
Lets say i have created a compound index {a:1, b:1, c:1}. This should make according to
http://docs.mongodb.org/manual/core/indexes/#compound-indexes
the following queries fast.
db.test.find({a:"a", b:"b",c:"c"})
db.test.find({a:"a", b:"b"})
db.test.find({a:"a"})
As i understand it the order of the query is very important, but is it only that explicit subset of {a:"a", b:"b",c:"c"} order that is important?
Lets say i do a query
db.test.find({d:"d",e:"e",a:"a", b:"b",c:"c"})
or
db.test.find({a:"a", b:"b",c:"c",d:"d",e:"e"})
Will these render useless for that specific compound index?

Answer 1

Compound indexes in MongoDB work on a prefix mechanism whereby a and {a,b} would be considered prefixes, by order, of the compound index, however, the order of the fields in the query itself do not normally matter.
So lets take your examples:
db.test.find({d:"d",e:"e",a:"a", b:"b",c:"c"})
Will actually use an index:
db.ghghg.find({d:1,e:1,a:1,c:1,b:1}).explain()
{
"cursor" : "BtreeCursor a_1_b_1_c_1",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 1,
"nscannedObjectsAllPlans" : 2,
"nscannedAllPlans" : 2,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"a" : [
[
1,
1
]
],
"b" : [
[
1,
1
]
],
"c" : [
[
1,
1
]
]
},
"server" : "ubuntu:27017"
}
Since a and b are there.
db.test.find({a:"a", b:"b",c:"c",d:"d",e:"e"})
Depends upon the selectivity and cardinality of d and e. It will use the compound index but as to whether it will use it effectively in a such a manner that allows decent performance of the query depends heavily upon what's in there.