Question

该集合是散列字段上的分片集合。以下查询应该定义为indexOnly，但解释说明不然。

db.collection.ensureIndex({field : "hashed"})
db.collection.ensureIndex({field : 1, "field2" : 1, "field3" : 1})

db.collection.find(
{
    field : 100
}
,{field : 1, _id : 0}
)
//.hint({    "field" : 1,    "field2" : 1,    "field3" : 1})
//.hint({    "field" : "hashed"})
.explain()

"cursor" : "BtreeCursor field_hashed",
"nscannedObjects" : 1,
"nscanned" : 1,
"indexOnly" : false,

我测试过提示两个索引，但没有一个生成覆盖查询。我将不胜感激任何帮助或建议。

解释（）：

{
    "clusteredType" : "ParallelSort",
    "shards" : {
        "repset12" : [ 
            {
                "cursor" : "BtreeCursor field_hashed",
                "isMultiKey" : false,
                "n" : 1,
            "nscannedObjects" : 1,
            "nscanned" : 1,
            "nscannedObjectsAllPlans" : 2,
            "nscannedAllPlans" : 2,
            "scanAndOrder" : false,
            "indexOnly" : false,
            "nYields" : 0,
            "nChunkSkips" : 0,
            "millis" : 0,
            "indexBounds" : {
                "field" : [ 
                    [ 
                        NumberLong(5346856657151215906), 
                        NumberLong(5346856657151215906)
                    ]
                ]
            },
            "server" : "server",
            "filterSet" : false,
            "stats" : {
                "type" : "PROJECTION",
                "works" : 3,
                "yields" : 0,
                "unyields" : 0,
                "invalidates" : 0,
                "advanced" : 1,
                "needTime" : 0,
                "needFetch" : 0,
                "isEOF" : 1,
                "children" : [ 
                    {
                        "type" : "KEEP_MUTATIONS",
                        "works" : 3,
                        "yields" : 0,
                        "unyields" : 0,
                        "invalidates" : 0,
                        "advanced" : 1,
                        "needTime" : 0,
                        "needFetch" : 0,
                        "isEOF" : 1,
                        "children" : [ 
                            {
                                "type" : "SHARDING_FILTER",
                                "works" : 2,
                                "yields" : 0,
                                "unyields" : 0,
                                "invalidates" : 0,
                                "advanced" : 1,
                                "needTime" : 0,
                                "needFetch" : 0,
                                "isEOF" : 1,
                                "chunkSkips" : 0,
                                "children" : [ 
                                    {
                                        "type" : "FETCH",
                                        "works" : 1,
                                        "yields" : 0,
                                        "unyields" : 0,
                                        "invalidates" : 0,
                                        "advanced" : 1,
                                        "needTime" : 0,
                                        "needFetch" : 0,
                                        "isEOF" : 1,
                                        "alreadyHasObj" : 0,
                                        "forcedFetches" : 0,
                                        "matchTested" : 1,
                                        "children" : [ 
                                            {
                                                "type" : "IXSCAN",
                                                "works" : 1,
                                                "yields" : 0,
                                                "unyields" : 0,
                                                "invalidates" : 0,
                                                "advanced" : 1,
                                                "needTime" : 0,
                                                "needFetch" : 0,
                                                "isEOF" : 1,
                                                "keyPattern" : "{ field: \"hashed\" }",
                                                "boundsVerbose" : "field #0['field']: [5346856657151215906, 5346856657151215906]",
                                                "isMultiKey" : 0,
                                                "yieldMovedCursor" : 0,
                                                "dupsTested" : 0,
                                                "dupsDropped" : 0,
                                                "seenInvalidated" : 0,
                                                "matchTested" : 0,
                                                "keysExamined" : 1,
                                                "children" : []
                                            }
                                        ]
                                    }
                                ]
                            }
                        ]
                    }
                ]
            }
        }
    ]
},
"cursor" : "BtreeCursor field_hashed",
"n" : 1,
"nChunkSkips" : 0,
"nYields" : 0,
"nscanned" : 1,
"nscannedAllPlans" : 2,
"nscannedObjects" : 1,
"nscannedObjectsAllPlans" : 2,
"millisShardTotal" : 0,
"millisShardAvg" : 0,
"numQueries" : 1,
"numShards" : 1,
"indexBounds" : {
    "field" : [ 
        [ 
            NumberLong(5346856657151215906), 
            NumberLong(5346856657151215906)
        ]
    ]
},
"millis" : 1
}

Answer 1

与MongoDB 2.6一样，您无法获得完全覆盖的分片查询，因为有一个额外的查询来检查有问题的分片是否拥有该文档（请参阅MongoDB问题跟踪器中的SERVER-5022）。

mongos路由器过滤在分片上找到但根据分片群集元数据不应该存在的文档。

如果出现以下情况，文档可以存在于多个分片上：

正在进行chunk migration：文档从捐赠者分片复制到目标分片，并且在块迁移成功完成之前不会从捐赠者分片中删除。
文件是孤儿＆＃34;由于迁移失败或清理不完整而在碎片上。 MongoDB 2.6中有cleanupOrphaned admin command，可以针对分片mongod运行以删除孤立的文档。

这个涵盖的查询限制在MongoDB文档的Limits: Covered Queries in Sharded Clusters部分中注明，但也应在Creating Covered Queries的教程中突出显示。我已经提出DOCS-3820以使其更加明显。

无法在MongoDB中获取有关分片集合的覆盖查询

1 个答案: