Question

Today I have noticed that the order in which the $lt and $gt operators are given seem to matter in MongoDB 2.0.2.

I have a database of games. “player” is an array of two strings representing both players, “endedAtMS” is a timestamp when the game has ended. I have created this index:

db.games.ensureIndex({player:1,endedAtMS:-1})

To get 30 of my games which were finished in a certain time range, ordered by the time the games where finished, I do:

db.games.find({ "player" : "Stefan" , 
                "endedAtMS" : { "$lt" : 1321284969946 , 
                                "$gt" : 1301284969946}}).
         sort({endedAtMS:-1}).
         limit(30).
         explain()

{
    "cursor" : "BtreeCursor player_1_endedAtMS_-1",
    "nscanned" : 30,
    "nscannedObjects" : 30,
    "n" : 30,
    "millis" : 0,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "isMultiKey" : true,
    "indexOnly" : false,
    "indexBounds" : {
        "player" : [
            [
                "Stefan",
                "Stefan"
            ]
        ],
        "endedAtMS" : [
            [
                1321284969946,
                -1.7976931348623157e+308
            ]
        ]
    }
}

All seems to work fine. However when I change the order of $lt and $gt in the query above I get this:

db.games.find({ "player" : "Stefan" , 
                "endedAtMS" : { "$gt":1301284969946, 
                                "$lt" : 1321284969946}}).
         sort({endedAtMS:-1}).
         limit(30).
         explain()

{
    "cursor" : "BtreeCursor player_1_endedAtMS_-1",
    "nscanned" : 126,
    "nscannedObjects" : 126,
    "n" : 30,
    "millis" : 1,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "isMultiKey" : true,
    "indexOnly" : false,
    "indexBounds" : {
        "player" : [
            [
                "Stefan",
                "Stefan"
            ]
        ],
        "endedAtMS" : [
            [
                1.7976931348623157e+308,
                1301284969946
            ]
        ]
    }
}

As you can see 126 docs need to be scanned to get the 30 docs for the result. If you take a look at the indexBounds in the explain output it seems that only the first operator is used to limit the search space in the index.

What do I miss? Why is Mongo only using one operator to limit the search space?

Answer 1

This is a known issue. The short answer is that it has to do with the fact that a multikey index is used (“player” is an array), and the index cannot be constrained on both upper and lower bounds.

This is explained in more detail in the Jira case: https://jira.mongodb.org/browse/SERVER-4155 – “Index bound incorrect?”

There is an open Jira ticket to improve this behavior: https://jira.mongodb.org/browse/SERVER-4180 – “Wrong indexbounds picked for a date range query (regression)” which is slated to be released in version 2.1.2 (this version is subject to change). Please vote for it!