Analyzing and Improving Database Performance - MongoDB High Availability - page 127

Introducing other analytics methods
MongoDB provides the explain() function, which provides you a detailed procedure
for the current operator. For instance, consider following command:
db.testcollection.find().sort({x: -1}).explain()
The preceding command will execute the find operator and sort the records descending
from the x field. Then, the explain() function shows an output as follows:
{
"cursor": "BasicCursor",
"isMultiKey": false,
"n": 100000,
"nscannedObjects": 100000,
"nscanned": 100000,
"nscannedObjectsAllPlans": 100000,
"nscannedAllPlans": 100000,
"scanAndOrder": true,
"indexOnly": false,
"nYields": 1563,
"nChunkSkips": 0,
"millis": 406,
"server": "mongotest:27017",
"filterSet": false
}
In the preceding JSON format, the n field is an integer that reflects the number of affected
records. Also, the indexOnly field is Boolean and returns true only when returned
fields are already indexed.
Moreover, the millis field is an integer value that shows the time spent in milliseconds
to complete the operator.
Note
In order to find more information about the explain() function, please visit ht-
tp://docs.mongodb.org/manual/reference/method/cursor.explain/ .