From this blog, we start to talk about the index in mongo Database, which is similar to the traditional database. Generally speaking, if the index need to be created in the traditional database, so does MongoDB.  In MongoDB ,the field '_id ' has been set index by default and this index is so special that it cannot be deleted except for Capped Collections.

Before studying the index, now we create 10000 test records as follows.

1 Create index

In mongodb, using the function ensureIndex() to create tne index, for example: 

      db.test.ensureIndex({name : 1}) means to create index for name

2. Use index 

generally speaking, there is five way to create index based on  the practice condition. 

2.1 Common Index

query the result before creating index and after. 

             Some explanation of result field:

Cursor: value [BasicCursor or BtreeCursor], the later explain this query has used index.

nscanned: the number of index of having scan. 

n : return the number of document, namely return the line 

millis: the total time of finishing this query, unit millinus seconds.

indexBounds: if not null, it will describe the index item,