当 INDEX 匹配时，MongoDB 阶段是否应该避免 FETCH？

Question

根据文档，当索引覆盖查询时，MongoDB 应跳过 FETCH 阶段。

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如果我理解正确的话，这句话解释了这种行为：

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覆盖查询 当索引覆盖查询时，MongoDB 既可以匹配查询条件，又可以仅使用索引键返回结果；\ni.e. MongoDB 不需要检查集合中的文档来\n返回结果。

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当索引覆盖查询时，解释结果有一个 IXSCAN 阶段，\n该阶段不是 FETCH 阶段的后代，并且在executionStats 中，\ntotalDocsExamined 为 0。

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在 MongoDB 的早期版本中，cursor.explain() 返回\nindexOnly 字段来指示索引是否覆盖查询。\n( https://docs.mongodb.com/manual/reference/explain-results/ )

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和这个

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这样，查询耗时不到 2 毫秒。由于索引\n\xe2\x80\x98 覆盖了\xe2\x80\x99 查询，MongoDB 能够匹配查询条件\n并仅使用索引返回结果钥匙；甚至不需要检查集合中的文档来返回结果。（如果您在执行计划中看到 IXSCAN 阶段不是 FETCH 阶段的子级，则索引 \xe2\x80\x98 覆盖\xe2\x80\x99 查询。）( https://studio3t.com /知识库/文章/mongodb-index-strategy/）

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但在测试场景中它不会发生：

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测试示例：

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    db.Test.insert({"Field1":"data on field1: 1","Field2":"data on field2: 1"});\n    db.Test.insert({"Field1":"data on field1: 2","Field2":"data on field2: 2"});\n    db.Test.insert({"Field1":"data on field1: 3","Field2":"data on field2: 3"});\n    db.Test.insert({"Field1":"data on field1: 4","Field2":"data on field2: 4"});\n    db.Test.insert({"Field1":"data on field1: 5","Field2":"data on field2: 5"});\n    db.Test.insert({"Field1":"data on field1: 6","Field2":"data on field2: 6"});\n

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在我创建了 Field2 的索引之后。

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db.Test.createIndex({"Field2":1})\n

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然后我查询集合：

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db.Test.find({"Field2":"data on field2: 5"}).explain("executionStats");\n

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IDXSCAN我期望一个不是阶段子级的阶段FETCH。但输出是这样的：

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[...]\n"winningPlan" : {\n            "stage" : "FETCH",\n            "inputStage" : {\n                "stage" : "IXSCAN",\n                "keyPattern" : {\n                    "Campo2" : 1.0\n                },\n                "indexName" : "Field2_1",\n                "isMultiKey" : false,\n                "multiKeyPaths" : {\n                    "Campo2" : []\n                },\n                "isUnique" : false,\n                "isSparse" : false,\n                "isPartial" : false,\n                "indexVersion" : 2,\n                "direction" : "forward",\n                "indexBounds" : {\n                    "Field2" : [ \n                        "[\\"data on field2: 5", \\"data on field2: 5\\"]"\n                    ]\n                }\n            }\n        },\n[...]\n

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有两个阶段：一个“阶段”：“FETCH”，及其子“阶段”：“IXSCAN”。

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谁能解释一下我的误解是什么？

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*** 关于投影

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使用投影运行查询时

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"winningPlan" : {\n            "stage" : "PROJECTION",\n            "transformBy" : {\n                "Campo2" : 1.0\n            },\n            "inputStage" : {\n                "stage" : "FETCH",\n                "inputStage" : {\n                    "stage" : "IXSCAN",\n                    "keyPattern" : {\n                        "Field2" : 1.0\n                    },\n                    "indexName" : "Field2_1",\n                    "isMultiKey" : false,\n                    "multiKeyPaths" : {\n                        "Campo2" : []\n                    },\n                    "isUnique" : false,\n                    "isSparse" : false,\n                    "isPartial" : false,\n                    "indexVersion" : 2,\n                    "direction" : "forward",\n                    "indexBounds" : {\n                        "Field2" : [ \n                            "[\\"data on field2: 5", \\"data on field2: 5\\"]"\n                        ]\n                    }\n                }\n            }\n        },\n

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Adam 的回答是：成功了！

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我意识到投影不应包含“_id”以避免FETCH.

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Answer 1

您的查询未指定投影，这意味着它将返回文档中的所有字段。这意味着{ Field2: 1 }索引不覆盖查询，因为它只包含单个字段。

下面的查询应该被完全覆盖并且不应该有 FETCH 阶段。请注意，投影明确排除了该_id字段，因为除非另有指定，否则该字段将包含在投影中：

    db.Test.find(
      {"Field2":"data on field2: 5"},
      {"Field2" : 1, "_id" : 0 }
    ).explain("executionStats");

输出：

{
    "queryPlanner" : {
        "plannerVersion" : 1,
        "namespace" : "foo.Test",
        "indexFilterSet" : false,
        "parsedQuery" : {
            "Field2" : {
                "$eq" : "data on field2: 5"
            }
        },
        "winningPlan" : {
            "stage" : "PROJECTION",
            "transformBy" : {
                "Field2" : 1,
                "_id" : 0
            },
            "inputStage" : {
                "stage" : "IXSCAN",
                "keyPattern" : {
                    "Field2" : 1
                },
                "indexName" : "Field2_1",
                "isMultiKey" : false,
                "multiKeyPaths" : {
                    "Field2" : [ ]
                },
                "isUnique" : false,
                "isSparse" : false,
                "isPartial" : false,
                "indexVersion" : 2,
                "direction" : "forward",
                "indexBounds" : {
                    "Field2" : [
                        "[\"data on field2: 5\", \"data on field2: 5\"]"
                    ]
                }
            }
        },
        "rejectedPlans" : [ ]
    },
    "executionStats" : {
        "executionSuccess" : true,
        "nReturned" : 1,
        "executionTimeMillis" : 1,
        "totalKeysExamined" : 1,
        "totalDocsExamined" : 0,
        "executionStages" : {
            "stage" : "PROJECTION",
            "nReturned" : 1,
            "executionTimeMillisEstimate" : 0,
            "works" : 2,
            "advanced" : 1,
            "needTime" : 0,
            "needYield" : 0,
            "saveState" : 0,
            "restoreState" : 0,
            "isEOF" : 1,
            "invalidates" : 0,
            "transformBy" : {
                "Field2" : 1,
                "_id" : 0
            },
            "inputStage" : {
                "stage" : "IXSCAN",
                "nReturned" : 1,
                "executionTimeMillisEstimate" : 0,
                "works" : 2,
                "advanced" : 1,
                "needTime" : 0,
                "needYield" : 0,
                "saveState" : 0,
                "restoreState" : 0,
                "isEOF" : 1,
                "invalidates" : 0,
                "keyPattern" : {
                    "Field2" : 1
                },
                "indexName" : "Field2_1",
                "isMultiKey" : false,
                "multiKeyPaths" : {
                    "Field2" : [ ]
                },
                "isUnique" : false,
                "isSparse" : false,
                "isPartial" : false,
                "indexVersion" : 2,
                "direction" : "forward",
                "indexBounds" : {
                    "Field2" : [
                        "[\"data on field2: 5\", \"data on field2: 5\"]"
                    ]
                },
                "keysExamined" : 1,
                "seeks" : 1,
                "dupsTested" : 0,
                "dupsDropped" : 0,
                "seenInvalidated" : 0
            }
        }
    },
    "serverInfo" : {
    ...
    },
    "ok" : 1,
    ...
}