MongoDB aggregation running two group operations at once

Question

Given:

A collection where I have some columns where for two of them I want to get all used values

Currently I'm doing two aggregations like below (simplified)

db['myCollection'].aggregate(
  [{$group: {"_id":"$firstCol"}}  ]
);

and

db['myCollection'].aggregate(
  [{$group: {"_id":"$secondCol"}}  ]
);

But it feels like a waste of resources to run two aggregations. Is there a way to get both grouped results at once?

I thought about combining them and later on split it in code.

It is unclear what you are asking here. Do you want to group by "firstCol" and "secondCol"? Can you provide sample document with the expected result? — Sede
– Sede, Commented Jul 14, 2016 at 8:32

DAXaholic · Accepted Answer · 2016-07-14 07:17:59Z

1

You could do it like so with the aid of the $addToSet operator

db['myCollection'].aggregate([
    {
        $group: {
            _id: null,
            col1: {
                $addToSet: "$firstCol"
            },
            col2: {
                $addToSet: "$secondCol"
            }
        }
    }
])

If you have indexes on the target fields then I would choose separate calls with distinct, as this can take advantage of those and probably result in less workload / faster execution times.

db[myCollection].distinct('firstCol')

answered Jul 14, 2016 at 7:17

DAXaholic

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DAXaholic Over a year ago

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