MongoDB查询不对带有文本字段的复合索引使用前缀 [英] Mongodb query does not use prefix on compound index with text field

问题描述

我在收藏夹上创建了以下索引:

I've created the following index on my collection:

db.myCollection.createIndex({
  user_id: 1,
  name: 'text'
})

如果我尝试查看包含两个字段的查询的执行计划，如下所示:

If I try to see the execution plan of a query containing both fields, like this:

db.getCollection('campaigns').find({ 
    user_id: ObjectId('xxx')
   ,$text: { $search: 'bla' } 
}).explain('executionStats')

我得到以下结果:

...
"winningPlan" : {
    "stage" : "TEXT",
    "indexPrefix" : {
        "user_id" : ObjectId("xxx")
    },
    "indexName" : "user_id_1_name_text",
    "parsedTextQuery" : {
        "terms" : [ 
            "e"
        ],
        "negatedTerms" : [],
        "phrases" : [],
        "negatedPhrases" : []
    },
    "inputStage" : {
        "stage" : "TEXT_MATCH",
        "inputStage" : {
            "stage" : "TEXT_OR",
            "inputStage" : {
                "stage" : "IXSCAN",
                "keyPattern" : {
                    "user_id" : 1.0,
                    "_fts" : "text",
                    "_ftsx" : 1
                },
                "indexName" : "user_id_1_name_text",
                "isMultiKey" : true,
                "isUnique" : false,
                "isSparse" : false,
                "isPartial" : false,
                "indexVersion" : 1,
                "direction" : "backward",
                "indexBounds" : {}
            }
        }
    }
}
...

如文档所述， MongoDB可以使用索引前缀来执行索引查询.

As stated in the documentation, MongoDB can use index prefixes to perform indexed queries.

由于user_id是上述索引的前缀，所以我希望仅通过user_id进行的查询会使用该索引，但是如果我尝试以下操作:

Since user_id is a prefix for the index above, I'd expect that a query only by user_id would use the index, but if I try the following:

db.myCollection.find({ 
    user_id: ObjectId('xxx')
}).explain('executionStats')

我得到:

...
"winningPlan" : {
    "stage" : "COLLSCAN",
    "filter" : {
        "user_id" : {
            "$eq" : ObjectId("xxx")
        }
    },
    "direction" : "forward"
},
...

因此，它根本不使用索引并执行完整的集合扫描.

So, it is not using the index at all and performing a full collection scan.

推荐答案

通常，MongoDB可以使用索引前缀来支持查询，但是包括地理空间或文本字段的复合索引是

In general MongoDB can use index prefixes to support queries, however compound indexes including geospatial or text fields are a special case of sparse compound indexes. If a document does not include a value for any of the text index field(s) in a compound index, it will not be included in the index.

为了确保正确的结果对于前缀搜索，将在稀疏复合索引上选择替代查询计划:

In order to ensure correct results for a prefix search, an alternative query plan will be chosen over the sparse compound index:

如果索引稀疏会导致查询和排序操作的结果集不完整，则MongoDB将不会使用该索引，除非hint()明确指定索引.

If a sparse index would result in an incomplete result set for queries and sort operations, MongoDB will not use that index unless a hint() explicitly specifies the index.

在MongoDB 3.4.5中设置一些测试数据以演示潜在问题:

Setting up some test data in MongoDB 3.4.5 to demonstrate the potential problem:

db.myCollection.createIndex({ user_id:1, name: 'text' }, { name: 'myIndex'})

// `name` is a string; this document will be included in a text index
db.myCollection.insert({ user_id:123, name:'Banana' })

// `name` is a number; this document will NOT be included in a text index
db.myCollection.insert({ user_id:123, name: 456 })

// `name` is missing; this document will NOT be included in a text index
db.myCollection.insert({ user_id:123 })

然后，强制使用复合文本索引:

Then, forcing the compound text index to be used:

db.myCollection.find({user_id:123}).hint('myIndex')

结果仅包含带有索引文本字段name的单个文档，而不是预期的三个文档:

The result only includes the single document with the indexed text field name, rather than the three documents that would be expected:

{
  "_id": ObjectId("595ab19e799060aee88cb035"),
  "user_id": 123,
  "name": "Banana"
}

该异常应在MongoDB文档中更清楚地突出显示；在MongoDB问题跟踪器中监视/更新 DOCS-10322 .

This exception should be more clearly highlighted in the MongoDB documentation; watch/upvote DOCS-10322 in the MongoDB issue tracker for updates.

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