mongodb

来源：互联网发布：国家统计年鉴数据库编辑：程序博客网时间：2024/04/29 07:23

 db.testData.find({"groups":{$gt:10,$lt:15}}).explain(){        "cursor" : "BasicCursor",        "isMultiKey" : false,        "n" : 4,        "nscannedObjects" : 10000,        "nscanned" : 10000,        "nscannedObjectsAllPlans" : 10000,        "nscannedAllPlans" : 10000,        "scanAndOrder" : false,        "indexOnly" : false,        "nYields" : 0,        "nChunkSkips" : 0,        "millis" : 10,        "indexBounds" : {        },        "server" : "ubuntu:27017"}> db.testData.ensureIndex({"groups":1})> db.testData.find({"groups":{$gt:10,$lt:15}}).explain(){        "cursor" : "BtreeCursor groups_1",        "isMultiKey" : false,        "n" : 4,        "nscannedObjects" : 4,        "nscanned" : 4,        "nscannedObjectsAllPlans" : 4,        "nscannedAllPlans" : 4,        "scanAndOrder" : false,        "indexOnly" : false,        "nYields" : 0,        "nChunkSkips" : 0,        "millis" : 0,        "indexBounds" : {                "groups" : [                        [                                10,                                15                        ]                ]        },        "server" : "ubuntu:27017"}> db.testData.find({"groups":{$gt:10,$lt:15}}).explain(){        "cursor" : "BtreeCursor groups_1",        "isMultiKey" : false,        "n" : 4,        "nscannedObjects" : 4,        "nscanned" : 4,        "nscannedObjectsAllPlans" : 4,        "nscannedAllPlans" : 4,        "scanAndOrder" : false,        "indexOnly" : false,        "nYields" : 0,        "nChunkSkips" : 0,        "millis" : 0,        "indexBounds" : {                "groups" : [                        [                                10,                                15                        ]                ]        },        "server" : "ubuntu:27017"}

mongodb查询加索引减索引的查询效果差别：

nscanned体现了Mongo扫描字段索引的条数，而nscannedObjects则为最终结果中查询过的文档数目。n则表示了返回文档的数目。nscannedObjects至少包含了所有的返回文档，即使Mongo明确了可以通过查看绝对匹配文件的索引。因此可以得出nscanned >= nscannedObjects >= n。

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