从集合中检索最后一个文档时如何提高性能

Here is a statement where Limit() precedes Sort()...

var result Candle
dao.c.Find(bson.M{"symbol": "USD"}).Limit(1).Sort("-time").One(&result);

... and here is a statement where Limit() comes after Sort():

var result Candle
dao.c.Find(bson.M{"symbol": "USD"}).Sort("-time").Limit(1).One(&result);

Is there any performance difference between the statements above?

We might answer regarding the mgo package and MongoDB itself.

In mgo

The Query.Limit() and Query.Sort() methods just operate on the Query object locally, and once you setup the query, you execute it e.g. with Query.One() or Query.All(). The order in which you called the methods to set it up is not stored and does not matter.

In MongoDB

Quoting from MongoDB doc: Combine Cursor Methods:

The following statements chain cursor methods limit() and sort():

db.bios.find().sort( { name: 1 } ).limit( 5 )
db.bios.find().limit( 5 ).sort( { name: 1 } )

The two statements are equivalent; i.e. the order in which you chain the limit() and the sort() methods is not significant. Both statements return the first five documents, as determined by the ascending sort order on ‘name’.

So there is no difference, they are equivalent and thus have the same performance.