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"
}
>