Why Group is slower than $group from Aggregate?

Question

Why Group is slower than $group from Aggregate?

I tried to bench 3 methods to group data: native js (with underscore), group and Aggregate with $group

I uses these datas (genre/position' trees in Paris) (237 168 rows, 35Mo)

This is my script test and the result is a bit surprising !

┌─────────────┬───────────────┐
│ Method      │ avg time (ms) │
├─────────────┼───────────────┤
│ Pure js     │ 897           │
├─────────────┼───────────────┤
│ Group       │ 3863          │
├─────────────┼───────────────┤
│ Aggregation │ 364           │
└─────────────┴───────────────┘

Why grouping with group is 10x slower than Aggregation ? For what is used "Group" ? And how can i optimise again my request ?

Thanks.

javascript
node.js
mongodb
grouping
aggregate

Answer 1

Group command uses the same framework as mapreduce and there are many resources for why MR is slower than aggregation framework. Main one is it runs in a separate JS thread, where agg framework runs natively on the server.

See details here MongoDB aggregation comparison: group(), $group and MapReduce