Question

I have a mongodb collection, “features”, having 3 fields: name, active, weight.
I will sort features by weight descending:

db.features.find({active:true},{name:1, weight:1}).sort({weight:-1})

for optimization, i create index for it:

db.features.ensureIndex({'active': 1, 'weight': -1})

I can see it works well when using explain() in query.

However, when i query it by weight ascending, i suppose the index i just created will not work and i need to create another index on weight ascending.
Query:

db.features.find({active:true},{name:1, weight:1}).sort({weight:1}).explain()

when i use explain() to show how index working, i find it prints out:

"cursor" : "BtreeCursor active_1_weight_-1 reverse",

does the index reverse mean the query is optimized by the index?

generally, do i need to create 2 index like ascending on weight and descending on weight if i will sort it by weight ascending in some case and descending in other cases?

Answer 1

As you can see from this document, when explain() outputs BtreeCursor, it means that an index was used. When an index is used, indexBounds will be set to indicate the key bounds for scanning in the index. However, if the putput showed BasicCursor, it indicates a table scan style operation.

So based on what you’ve said, from the explain() results, you can see that you’re using a BTree Cursor on the index named active_1_weight_-1 and the reverse means that you’re iterating over the index in reverse order.

So no, you don’t need to create separate indexes.