Nosql Mongodb之旅(19)—MongoDB explain执行计划

MongoDB 提供了一个 explain 命令让我们获知系统如何处理查询请求。利用 explain 命令，我们可以很好地观察系统如何使用索引来加快检索，同时可以针对性优化索引。

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1. >db.t5.ensureIndex({name:1})
2. >db.t5.ensureIndex({age:1})
3. >db.t5.find({age:{$gt:45}},{name:1}).explain()
4. {
5. "cursor":"BtreeCursorage_1",
6. "nscanned":0,
7. "nscannedObjects":0,108); list-style:decimal-leading-zero outside; line-height:18px; margin:0px!important; padding:0px 3px 0px 10px!important"> "n":0,108); list-style:decimal-leading-zero outside; line-height:18px; margin:0px!important; padding:0px 3px 0px 10px!important"> "millis":0,108); list-style:decimal-leading-zero outside; line-height:18px; margin:0px!important; padding:0px 3px 0px 10px!important"> "nYields":0,108); list-style:decimal-leading-zero outside; line-height:18px; margin:0px!important; padding:0px 3px 0px 10px!important"> "nChunkSkips":0,108); list-style:decimal-leading-zero outside; line-height:18px; margin:0px!important; padding:0px 3px 0px 10px!important"> "isMultiKey":false,108); list-style:decimal-leading-zero outside; line-height:18px; margin:0px!important; padding:0px 3px 0px 10px!important"> "indexOnly":false,108); list-style:decimal-leading-zero outside; line-height:18px; margin:0px!important; padding:0px 3px 0px 10px!important"> "indexBounds":{
8. "age":[
9. [
10. 45,108); list-style:decimal-leading-zero outside; line-height:18px; margin:0px!important; padding:0px 3px 0px 10px!important"> 1.7976931348623157e+308
11. ]
12. ]
13. }
14. }

字段说明:
cursor: 返回游标类型(BasicCursor 或 BtreeCursor)
nscanned: 被扫描的文档数量
n: 返回的文档数量
millis: 耗时(毫秒)
indexBounds: 所使用的索引