ing.ensureIndex({name: 1, age : 1});
如果要复合唯一索引，则：
db.Indexing.ensureIndex({name: 1, age : 1}, {unique: true})
同样要注意的是，复合索引，要一起用才有效果，比如：
db.Indexing.find({name: "Denis"}).explain()，只按一个NAME，索引使用情况为：
{
"cursor" : "BtreeCursor name_1_age_1",
"isMultiKey" : false,
"n" : 1,
"nscannedObjects" : 1,
"nscanned" : 1,
"nscannedObjectsAllPlans" : 1,
"nscannedAllPlans" : 1,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"name" : [
[
"Denis",
"Denis"
]
],
"age" : [
[
{
"$minElement" : 1
},
{
"$maxElement" : 1
}
]
]
},
"server" : "Denis:27017"
}
如果db.Indexing.find({age: "20"}).explain()，则没能使用索引
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 0,
"nscannedObjects" : 5,
"nscanned" : 5,
"nscannedObjectsAllPlans" : 5,
"nscannedAllPlans" : 5,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
},
"server" : "Denis:27017"
}