MongoDB索引执行集合扫描而不是索引扫描

Question

我有一个在名称字段上具有唯一索引的集合。这是getIndexes（）的输出：

{
        "v" : 2,
        "unique" : true,
        "key" : {
            "name" : 1
        },
        "name" : "name_1",
        "ns" : "xxx",
        "sparse" : true,
        "background" : true,
        "collation" : {
            "locale" : "en",
            "caseLevel" : false,
            "caseFirst" : "off",
            "strength" : 2,
            "numericOrdering" : false,
            "alternate" : "non-ignorable",
            "maxVariable" : "punct",
            "normalization" : false,
            "backwards" : false,
            "version" : "57.1"
        }
    }

现在，当我执行.find(name: 'B').explain('executionStats')时，在输出中看到它执行了集合扫描而不是索引扫描。 explain('executionStats')的输出：

"executionStats" : {
        "executionSuccess" : true,
        "nReturned" : 1,
        "executionTimeMillis" : 0,
        "totalKeysExamined" : 0,
        "totalDocsExamined" : 26,
        "executionStages" : {
            "stage" : "COLLSCAN",
            "filter" : {
                "name" : {
                    "$eq" : "B"
                }
            },
            "nReturned" : 1,
            "executionTimeMillisEstimate" : 0,
            "works" : 28,
            "advanced" : 1,
            "needTime" : 26,
            "needYield" : 0,
            "saveState" : 0,
            "restoreState" : 0,
            "isEOF" : 1,
            "direction" : "forward",
            "docsExamined" : 26
        }
    }

我不明白为什么它不使用索引来扫描文档。请注意，docExamined中的计数26是集合中文档的总数。