How to 'explain' a runCommand?

Question

Is there a way to run explain with runCommand ? I have the following query:

db.runCommand({geoNear:"Locations", near:[50,50], spherical:true})

How can I run an explain on it? I want to get the execution time.

Answer 1

As far as I know, explain is a method on cursors. You, however, can enable the integrated mongodb profiler :

db.setProfilingLevel(2); // log all operations
db.setProfilingLevel(1, 50); // log all operations longer than 50msecs

This will log details like nscanned , nreturned to a capped collection called system.profile , but does not provide as much detail as an explain() call does.

In this case , however, I think it might be possible to change the runCommand to a $near -query instead? That would give you full access to explain .

Answer 2

I guess we can't do explain for runCommand. Some of the runCommand give you the stats automatically ( eg. distinct command : db.runCommand({distinct : 'test', key : 'a'}) )

But, you can take help of the query profiler.

db.setProfilingLevel(2)

Once you run that query, switch off the profiler, and check the system.profile collection for this query.

Answer 3

According to explain docs you can simply:

db.runCommand({
    explain: {geoNear:"Locations", near:[50,50], spherical:true}
})

Answer 4

Both answers (from mnemosyn and Abhishek Kumar ) are correct.

However, if you just want to see the execution time (like me), this is provided in the results along with some extra stats:

...
"stats" : {
        "time" : 2689,
        "btreelocs" : 0,
        "nscanned" : 1855690,
        "objectsLoaded" : 979,
        "avgDistance" : 0.006218027001875209,
        "maxDistance" : 0.006218342348749806
},
"ok" : 1

Well, I should have looked closer before posting the question :).

Answer 5

Maybe not a runCommand but I use something like this:

expStats = function() { 
    var exp = db.collection.find({"stats.0.age" : 10},{"stats.age" : 1}).explain("allPlansExecution");

    print ("totalDocsExamined = "+exp.executionStats.totalDocsExamined);    
    print ("nReturned = "+exp.executionStats.nReturned);    
    return print ("execTime = "+exp.executionStats.executionTimeMillis);
}