问题描述
我无法在Mongodb的说明查询输出中获得"nscanned"和"nscannedObjects"之间的确切区别.
I cannot get the exact difference between "nscanned" and "nscannedObjects" in the Mongodb's explain query output.
关于 MongoDB解释文档我可以阅读:
nscannedObjects 扫描的文档数.
这两个字段之间有什么区别?更具体地讲，当我使用BtreeCursor(索引)进行查询时，这究竟意味着什么，而这两个字段具有两个不同的值，例如:
What's the different between these two fields?And more specific what does exactly mean when I have a query, which uses a BtreeCursor (an index), and these two fields have two different values, for example:
{
"cursor" : "BtreeCursor a_1_b_1",
"isMultiKey" : false,
"n" : 5,
"nscannedObjects" : 5,
"nscanned" : 9,
(...)
}
我知道什么是"已覆盖索引" .我想确切地了解查询在上面的示例中做了什么.它是否通过了9个元素(nscanned = 9)(其中"nscanned = 9")，它们全部都是索引条目，并且仅读取(检查")其中5个元素(nscannedObjects = 5)的值以生成结果集? /p>
I know what a "covered index" is.I would like to understand exactly what the query did in the example above.Did it pass through ("scanned") 9 elements (nscanned = 9), where all of them are index entries and read ("examined") the value of only 5 of them (nscannedObjects = 5) to produce the result set?
推荐答案
这意味着:
查询返回5个文档-n
从索引中扫描了9个文档-nscanned
然后从集合中读取5个完整文档-nscannedObjects
This means that :
The query returned 5 documents - n
scanned 9 documents from the index - nscanned
and then read 5 full documents from the collection - nscannedObjects
类似的示例在:
http://docs.mongodb.org/manual/reference/method/cursor.explain /#cursor.explain
Similar example is given at :
http://docs.mongodb.org/manual/reference/method/cursor.explain/#cursor.explain
这篇关于Mongodb中的explain():"nscanned"和"nscanned"之间的区别和"nscannedObjects"的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！