MongoDB Learning Note 24 explain and hint functions for MongoDB

First, explain function

The explain function can provide a large number of query-related information, and if it is a slow query, it is the most important diagnostic tool. For example:

Query on fields that have an index:

> db.post.find ({"loc.city": "NY"}). Explain ()    {          "cursor"  :  "Btreecursor loc.city_1",          "Ismultikey"  : false,         "n"  : 0,          "Nscannedobjects"  : 0,          "nscanned"  : 0,         "Nscannedobjectsallplans"  : 0,         "Nscannedallplans"  : 0,          "Scanandorder"  : false,          "IndexOnly"  : false,         "Nyields"  :  0,         "Nchunkskips"  : 0,          "Millis"  : 1,         "Indexbounds"  : {              "Loc.city"  : [                 [                      "NY",                      "NY"                  ]             ]        },          "Server"  :  "localhost.localdomain:27017",          "Filterset"  : false    }     >

The

Queries on fields that do not have an index:

> db.post.find ({"Name": "Joe"}). Explain ()    {          "cursor"  :  "basiccursor",         "Ismultikey"  :  false,         "n"  : 2,          "Nscannedobjects"  : 15,         "nscanned"  : 15,         "Nscannedobjectsallplans"  : 15,          "Nscannedallplans"  : 15,          "Scanandorder"  : false,         " IndexOnly " : false,        " Nyields " : 0,          "Nchunkskips"  : 0,          "Millis"  : 0,         "Server"  :  "localhost.localdomain:27017",          "Filterset"  : false    }     >

In contrast to the above two queries, the interpretation of the fields in the explain results:

"Cursor": "Basiccursor" indicates that the query does not use an index; "Btreecursor loc.city_1" means that the index on loc.city is used;

"Ismultikey" indicates whether a multi-key index is used;

"N": the number of documents returned by this query;

"Nscannedobjects": Indicates the number of times the actual document was actually found on the disk according to the index pointer;

"Nscanned": If there is no index, this number is the number of indexed entries searched;

"Scanandorder": Whether the result set is sorted;

"IndexOnly": whether the index can be used to complete the index;

"Nyields": if there is a write operation during the query, the query will be paused; This field represents the number of pauses in the query due to write operations;

"Millis": the number of times the query is spent, the smaller the number, the higher the efficiency of the query;

"Indexbounds": This field describes the usage of the index and gives the scope of the index traversal.

"Filterset": whether to use and index filter;

Second, hint function

If you find that the index used by MongoDB is inconsistent with the index you hope for. , you can use the hit function to force MongoDB to use a specific index. For example

>db.users.find ({"Age": 1, "username":/.*/}). Hint ({"username": 1, "Age": 1})

This article is from the "Margin with Wish" blog, please be sure to keep this source http://281816327.blog.51cto.com/907015/1601477

MongoDB Learning Note 24 explain and hint functions for MongoDB