当$ match键不在索引中时，mongodb聚合非常慢（tablescan吗？）

Question

db.collection.aggregate(

{  "$match" : {
        "key" : "mykey"

    }
}, 
{
    "$sort" : {
        "time" : -1
    }
},
{
    "$limit" : 1 
}

)

example document:

{
   key: "key1",
   time: ISODate("2014-07-04T20:04:46.904Z")
}

indexes

"time" : -1
"key" : 1,
"_id" : 1

when "mykey" exists in the collection the query takes 30ms, when "mykey" does not exist it takes 10s, explain tells me indexes are used. This is a capped collection, therefor it usually occurs that "keys" are missing. Why does it take that long. btw. Mongodb 2.4

further exploration:

removing the index for the sort reduces the lookup time:

explain for aggregate with and without index on the sort field shows that with index the sort gets executed at the start of the pipeline, without index on sort it gets executed as last step of the pipeline

Answer 1

Your query is equality on key and sort on time which means that you are using the wrong index for this (your index is on time:1, key:1 in essence).

The order of fields for the query you are running should be key:1, time:1 (as first two fields) in order to have effective help from it. With that index, the matched key value can be jumped to directly, and then if there are multiple time values for that key then they are sorted and the highest one can be immediately fetched. If key is not found in the index then you're done.

As it is, the query is forced to scan all time values in Index (leading field) so that when you find the first matching key you'll be able to return. When key you are looking for doesn't exist, the query ends up scanning through the entire index before it can return.