MongoDB db.collection. ensureIndex 和 db.collection.createIndex

注意从mongoDB 3.0开始ensureIndex被废弃，今后都仅仅是db.collection.createIndex的一个别名。

db.collection.createIndex主要分成2部分： KEY和OPTION。

KEY:

> db.dbdao_product.ensureIndex({x:1});
{
	"createdCollectionAutomatically" : false,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}


> db.dbdao_product.find({x:200});
{ "_id" : ObjectId("5524face073d738e116afc2c"), "x" : 200, "name" : "askmac.cn", "name1" : "askmac.cn", "name2" : "askmac.cn", "name3" : "askmac.cn" }
{ "_id" : ObjectId("5524face073d738e116afc2d"), "x" : 200, "name" : "askmac.cn", "name1" : "askmac.cn", "name2" : "askmac.cn", "name3" : "askmac.cn" }
> 
> db.dbdao_product.find({x:200}).explain();
{
	"queryPlanner" : {
		"plannerVersion" : 1,
		"namespace" : "test.dbdao_product",
		"indexFilterSet" : false,
		"parsedQuery" : {
			"x" : {
				"$eq" : 200
			}
		},
		"winningPlan" : {
			"stage" : "FETCH",
			"inputStage" : {
				"stage" : "IXSCAN",
				"keyPattern" : {
					"x" : 1
				},
				"indexName" : "x_1",
				"isMultiKey" : false,
				"direction" : "forward",
				"indexBounds" : {
					"x" : [
						"[200.0, 200.0]"
					]
				}
			}
		},
		"rejectedPlans" : [ ]
	},
	"serverInfo" : {
		"host" : "dbdao-Inspiron-560s",
		"port" : 27017,
		"version" : "3.0.2",
		"gitVersion" : "6201872043ecbbc0a4cc169b5482dcf385fc464f"
	},
	"ok" : 1
}

dropDups 参数在mongodb 3.0.2中被废弃， 该参数的用意是在创建unique的索引时若遇到duplicate重复document，则仅仅保持_ID最小的哪一个document:
A unique index cannot be created on a key that has duplicate values. If you would like to create the index anyway, keeping the first document the database indexes and deleting all subsequent documents that have duplicate values, add the dropDups option.

例如

MongoDB shell version: 3.0.2
connecting to: test


> db.dupme.insert({x:1,y:1});
WriteResult({ "nInserted" : 1 })
> db.dupme.insert({x:1,y:1});
WriteResult({ "nInserted" : 1 })
> db.dupme.insert({x:1,y:1});
WriteResult({ "nInserted" : 1 })
> db.dupme.insert({x:1,y:1});
WriteResult({ "nInserted" : 1 })
> 
> db.dupme.find();
{ "_id" : ObjectId("552524627501a28814fdddc7"), "x" : 1, "y" : 1 }
{ "_id" : ObjectId("552524637501a28814fdddc8"), "x" : 1, "y" : 1 }
{ "_id" : ObjectId("552524657501a28814fdddc9"), "x" : 1, "y" : 1 }
{ "_id" : ObjectId("552524667501a28814fdddca"), "x" : 1, "y" : 1 }
> 




> db.dupme.ensureIndex({x:1},{unique:true, dropDups:true});
{
	"createdCollectionAutomatically" : false,
	"numIndexesBefore" : 1,
	"errmsg" : "exception: E11000 duplicate key error collection: test.dupme index: x_1 dup key: { : 1.0 }",
	"code" : 11000,
	"ok" : 0
}


> db.dupme.ensureIndex({x:1},{unique:true, dropDups:true});
{
	"createdCollectionAutomatically" : false,
	"numIndexesBefore" : 1,
	"errmsg" : "exception: E11000 duplicate key error collection: test.dupme index: x_1 dup key: { : 1.0 }",
	"code" : 11000,
	"ok" : 0
}

以上由于我们测试的版本是mongodb 3.0.2，所以直接忽略了dropDups参数。

在mongodb中 primary key主键是自动创建的且被分配给_id字段，用户不指定_id，那么mongodb会自动分配给你。_id字段总是被索引的且总是唯一的。 用户无法改变这一点，这是mongodb自己控制的。

> db.dbdao_foo.insert({a:3,b:2,c:1,d:4,e:2});
WriteResult({ "nInserted" : 1 })
> db.dbdao_foo.ensureIndex({a:1});
{
	"createdCollectionAutomatically" : false,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}
> db.dbdao_foo.ensureIndex({b:1});
{
	"createdCollectionAutomatically" : false,
	"numIndexesBefore" : 2,
	"numIndexesAfter" : 3,
	"ok" : 1
}
> db.dbdao_foo.ensureIndex({c:1});
{
	"createdCollectionAutomatically" : false,
	"numIndexesBefore" : 3,
	"numIndexesAfter" : 4,
	"ok" : 1
}

如上面的例子中 虽然针对a、b、c字段建了索引，但实际上主键总是_id。