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.