MongoDB limit() and sort() Optimization

Question

I am trying to understand why this is happening:

db.items.find({uid: {$in:[34, 54, 53,1,2,3,5,6,7]} }).limit(40).sort({_id:-1}).explain()

returns me:

"cursor" : "BtreeCursor _id_-1_uid_1 multi",
"nscanned" : 167,
"nscannedObjects" : 40,
"n" : 40,
...

However, without the sort

db.items.find({uid: {$in:[34, 54, 53,1,2,3,5,6,7]} }).limit(40).explain()

returns me:

"cursor" : "BtreeCursor uid_1 multi",
"nscanned" : 40,
"nscannedObjects" : 40,
"n" : 40,
...

So what I dont get is, why when I add the sort, it scans more documents? I am sorting by id, also I have this index to try to improve things "_id_-1_uid_1" , but still it scans a lot of documents.

This is a huge issue in my case, because this is my local database, so just a few documents are been scanned, however in my live site, I have millons of documents been scanned for a reason which I dont know what it is.

Can anyone explain me what is going on?

What other indexes do you have? Perhaps try {uid:1, id:-1} instead... — Nic Cottrell
– Nic Cottrell, Commented Mar 14, 2012 at 10:43
I have tried both index, {uid:1, id:-1} and {id:-1,uid:1} ! and no success at all... — Arthur Neves
– Arthur Neves, Commented Mar 14, 2012 at 14:13

EwyynTomato · Accepted Answer · 2012-03-14 01:39:56Z

3

At present (2012/3/14), this is an unresolved bug :

https://jira.mongodb.org/browse/SERVER-3310

answered Mar 14, 2012 at 1:39

EwyynTomato

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1 Comment

Eric Caron Over a year ago

This bug is fixed as of the 2.6.x branch.

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