Question

我在每个文档中都有一个locked字段的集合。我有以下索引：

{
    locked : 1
}

当我通过计数操作执行此解释时

db.scheduled.find({locked: false}).explain({executionStats:1})
{
        "queryPlanner" : {
                "plannerVersion" : 1,
                "namespace" : "connectivity_recruiter.scheduled",
                "indexFilterSet" : false,
                "parsedQuery" : {
                        "locked" : {
                                "$eq" : false
                        }
                },
                "winningPlan" : {
                        "stage" : "FETCH",
                        "inputStage" : {
                                "stage" : "IXSCAN",
                                "keyPattern" : {
                                        "locked" : 1
                                },
                                "indexName" : "locked_1",
                                "isMultiKey" : false,
                                "direction" : "forward",
                                "indexBounds" : {
                                        "locked" : [
                                                "[false, false]"
                                        ]
                                }
                        }
                },

        .....

        "executionStats" : {
                "executionSuccess" : true,
                "nReturned" : 53045,
                "executionTimeMillis" : 299,
                "totalKeysExamined" : 53045,
                "totalDocsExamined" : 53045,
                "executionStages" : {
                        "stage" : "FETCH",
                        "nReturned" : 53045,
                        "executionTimeMillisEstimate" : 180,
                        "works" : 53046,
                        "advanced" : 53045,
                        "needTime" : 0,
                        "needFetch" : 0,
                        "saveState" : 417,
                        "restoreState" : 417,
                        "isEOF" : 1,
                        "invalidates" : 0,
                        "docsExamined" : 53045,
                        "alreadyHasObj" : 0,
                        "inputStage" : {
                                "stage" : "IXSCAN",
                                "nReturned" : 53045,
                                "executionTimeMillisEstimate" : 70,
                                "works" : 53046,
                                "advanced" : 53045,
                                "needTime" : 0,
                                "needFetch" : 0,
                                "saveState" : 417,
                                "restoreState" : 417,
                                "isEOF" : 1,
                                "invalidates" : 0,
                                "keyPattern" : {
                                        "locked" : 1
                                },
                                "indexName" : "locked_1",
                                "isMultiKey" : false,
                                "direction" : "forward",
                                "indexBounds" : {
                                        "locked" : [
                                                "[false, false]"
                                        ]
                                },
                                "keysExamined" : 53045,
                                "dupsTested" : 0,
                                "dupsDropped" : 0,
                                "seenInvalidated" : 0,
                                "matchTested" : 0
                        }
                },
        ...........
}

totalDocsExamined似乎表明正在扫描所有文档以便对其进行计数，而此操作可以仅使用索引来执行。怎么了？这是正常的吗？是否正在对该集合进行全面扫描？

由于

Answer 1

检查了所有返回的文档，索引仅用于过滤不检索文档。

如果你查看你的解释，你会注意到文档的数量等于所检查文档的数量。

为什么？在您获取整个文档时，您的索引只包含一个字段，mongodb的工作是查询键的索引，然后转到集合以获取文档。

当索引包含所有投影字段时，唯一的情况是不需要检查文档是否为覆盖查询。

点击此链接查看更多信息：https://docs.mongodb.com/manual/core/query-optimization/#covered-query

MongoDB可能会扫描文档以查找索引可能涉及的操作

1 个答案: