MongoDB-多键索引

多键索引

多键索引是专门针对数组字段的, 会为数组字段的每一个元素都创建一个索引。

?> 插入测试数据：

db.person.insert([
{name:'as', age:18, tags:['ahtml', 'bcss']},
{name:'bs', age:17, tags:['cjs', 'enode']},
{name:'cs', age:19, tags:[ 'dvue', 'freact']},
])

首先来看看我们没有创建多键索引之前的查询效果：

db.person.explain().find({'tags':{$in:['ahtml']}})

从如下结果集返回来看，是一个全表扫描的情况：

{
    "queryPlanner" : {
        "plannerVersion" : 1,
        "namespace" : "test.person",
        "indexFilterSet" : false,
        "parsedQuery" : {
            "tags" : {
                "$eq" : "ahtml"
            }
        },
        "winningPlan" : {
            "stage" : "COLLSCAN",
            "filter" : {
                "tags" : {
                    "$eq" : "ahtml"
                }
            },
            "direction" : "forward"
        },
        "rejectedPlans" : [ ]
    },
    "serverInfo" : {
        "host" : "LAPTOP-A8CAC6IT",
        "port" : 27017,
        "version" : "4.0.28",
        "gitVersion" : "af1a9dc12adcfa83cc19571cb3faba26eeddac92"
    },
    "ok" : 1
}

创建多键索引：

db.person.createIndex({tags:1})

再次进行查询：

db.person.explain().find({'tags':{$in:['ahtml']}})

返回结果：

{
    "queryPlanner" : {
        "plannerVersion" : 1,
        "namespace" : "test.person",
        "indexFilterSet" : false,
        "parsedQuery" : {
            "tags" : {
                "$eq" : "ahtml"
            }
        },
        "winningPlan" : {
            "stage" : "FETCH",
            "inputStage" : {
                "stage" : "IXSCAN",
                "keyPattern" : {
                    "tags" : 1
                },
                "indexName" : "tags_1",
                "isMultiKey" : true,
                "multiKeyPaths" : {
                    "tags" : [
                        "tags"
                    ]
                },
                "isUnique" : false,
                "isSparse" : false,
                "isPartial" : false,
                "indexVersion" : 2,
                "direction" : "forward",
                "indexBounds" : {
                    "tags" : [
                        "[\"ahtml\", \"ahtml\"]"
                    ]
                }
            }
        },
        "rejectedPlans" : [ ]
    },
    "serverInfo" : {
        "host" : "LAPTOP-A8CAC6IT",
        "port" : 27017,
        "version" : "4.0.28",
        "gitVersion" : "af1a9dc12adcfa83cc19571cb3faba26eeddac92"
    },
    "ok" : 1
}

很明显是通过索引进行命中对应的数据。

在我们创建了对应了多键索引之后，我们插入的测试数据对应的数据结构大概的体系结构如下可以进行稍微了解了解：

'ahtml' -> {name:'as', age:18, tags:['ahtml', 'bcss']}
'bcss' -> {name:'as', age:18, tags:['ahtml', 'bcss']}
'cjs' -> {name:'bs', age:17, tags:['cjs', 'enode']}
'dvue' -> {name:'cs', age:19, tags:[ 'dvue', 'freact']}
'enode' -> {name:'bs', age:17, tags:['cjs', 'enode']}
'freact' -> {name:'cs', age:19, tags:[ 'dvue', 'freact']}