Unreasonably slow MongoDB query, even though the query is simple and aligned to indexes

Question

I\'m running a MongoDB server (that\'s literally all it has running). The server has 64gb of RAM and 16 cores, plus 2TB of hard drive space to work with.

The Doc

Answer 1

Have you tried adding _id to your composite index. As you're using it as part of the query won't it still have to do a full table scan?

Answer 2

Bumped into a very similar problem, and the Indexing Advice and FAQ on Mongodb.org says, quote:

The range query must also be the last column in an index

So if you have the keys a,b and c and run db.ensureIndex({a:1, b:1, c:1}), these are the "guidelines" in order use the index as much as possible:

Good:

• find(a=1,b>2)

• find(a>1 and a<10)

• find(a>1 and a<10).sort(a)

Bad:

• find(a>1, b=2)

Only use a range query OR sort on one column. Good:

• find(a=1,b=2).sort(c)

• find(a=1,b>2)

• find(a=1,b>2 and b<4)

• find(a=1,b>2).sort(b)

Bad:

• find(a>1,b>2)

• find(a=1,b>2).sort(c)

Hope it helps!

/J

Answer 3

Ok I solved it. The culprit was "scanAndOrder": true, which suggested that the index wasn't being used as intended. The correct composite index has the the primary sort field first and then the fields being queried on.

{ "_id":1, "LastUpdated":1 }