Mongodb 简明教程

MongoDB - Advanced Indexing

我们已按照如下方式在名为 users 的集合中插入下列文档:

db.users.insert(
	{
		"address": {
			"city": "Los Angeles",
			"state": "California",
			"pincode": "123"
		},
		"tags": [
			"music",
			"cricket",
			"blogs"
		],
		"name": "Tom Benzamin"
	}
)

上述文档包含一个 address sub-document 和一个 tags array

Indexing Array Fields

假设我们希望根据用户的标签搜索用户文档。为此,我们将在集合中的 tags 数组上创建一个索引。

依次在数组上创建索引会针对其每个字段创建单独的索引项。因此,当我们在 tags 数组上创建索引时,将针对其值 music、cricket 和 blogs 创建单独的索引。

若要针对 tags 数组创建索引,请使用下列代码:

>db.users.createIndex({"tags":1})
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 2,
"numIndexesAfter" : 3,
"ok" : 1
}
>

创建索引后,我们可以像这样搜索集合的 tags 字段:

> db.users.find({tags:"cricket"}).pretty()
{
	"_id" : ObjectId("5dd7c927f1dd4583e7103fdf"),
	"address" : {
		"city" : "Los Angeles",
		"state" : "California",
		"pincode" : "123"
	},
	"tags" : [
		"music",
		"cricket",
		"blogs"
	],
	"name" : "Tom Benzamin"
}
>

若要验证使用了正确的索引,请使用下列 explain 命令:

>db.users.find({tags:"cricket"}).explain()

这会生成下列结果:

{
	"queryPlanner" : {
		"plannerVersion" : 1,
		"namespace" : "mydb.users",
		"indexFilterSet" : false,
		"parsedQuery" : {
			"tags" : {
				"$eq" : "cricket"
			}
		},
		"queryHash" : "9D3B61A7",
		"planCacheKey" : "04C9997B",
		"winningPlan" : {
			"stage" : "FETCH",
			"inputStage" : {
				"stage" : "IXSCAN",
				"keyPattern" : {
					"tags" : 1
				},
				"indexName" : "tags_1",
				"isMultiKey" : false,
				"multiKeyPaths" : {
					"tags" : [ ]
				},
				"isUnique" : false,
				"isSparse" : false,
				"isPartial" : false,
				"indexVersion" : 2,
				"direction" : "forward",
				"indexBounds" : {
					"tags" : [
						"[\"cricket\", \"cricket\"]"
					]
				}
			}
		},
		"rejectedPlans" : [ ]
	},
	"serverInfo" : {
		"host" : "Krishna",
		"port" : 27017,
		"version" : "4.2.1",
		"gitVersion" : "edf6d45851c0b9ee15548f0f847df141764a317e"
	},
	"ok" : 1
}
>

上述命令生成的 "cursor" : "BtreeCursor tags_1" 证实使用了正确的索引。

Indexing Sub-Document Fields

假设我们希望根据 city、state 和 pincode 字段搜索文档。由于所有这些字段都是 address 子文档字段的一部分,我们将针对子文档的所有字段创建一个索引。

若要针对子文档的所有三个字段创建索引,请使用下列代码:

>db.users.createIndex({"address.city":1,"address.state":1,"address.pincode":1})
{
	"numIndexesBefore" : 4,
	"numIndexesAfter" : 4,
	"note" : "all indexes already exist",
	"ok" : 1
}
>

在创建索引后,我们可以利用这个索引搜索任何子文档字段,如下: −

> db.users.find({"address.city":"Los Angeles"}).pretty()
{
	"_id" : ObjectId("5dd7c927f1dd4583e7103fdf"),
	"address" : {
		"city" : "Los Angeles",
		"state" : "California",
		"pincode" : "123"
	},
	"tags" : [
		"music",
		"cricket",
		"blogs"
	],
	"name" : "Tom Benzamin"
}

记住,查询表达式必须遵循指定的索引顺序。因此,上面创建的索引将支持以下查询 −

>db.users.find({"address.city":"Los Angeles","address.state":"California"}).pretty()
{
	"_id" : ObjectId("5dd7c927f1dd4583e7103fdf"),
	"address" : {
		"city" : "Los Angeles",
		"state" : "California",
		"pincode" : "123"
	},
	"tags" : [
		"music",
		"cricket",
		"blogs"
	],
	"name" : "Tom Benzamin"
}
>