problem.js

db.events.drop()

db.events.save({_id: "1", venue: "v1", act: "a1", dt: "2011-03-24 00:00"})
db.events.save({_id: "2", venue: "v1", act: "a1", dt: "2011-03-25 00:00"})

// the same act, playing at two venues at the same time? 
db.events.save({_id: "3", venue: "v2", act: "a1", dt: "2011-03-23 00:00"})
db.events.save({_id: "4", venue: "v2", act: "a1", dt: "2011-03-25 00:00"})

// Another act playing at venue 1
db.events.save({_id: "5", venue: "v1", act: "a2", dt: "2011-03-26 00:00"})
db.events.save({_id: "6", venue: "v1", act: "a2", dt: "2011-03-27 00:00"})


// 
// db.events.ensureIndex({venue:1, act:1, dt: 1})	
// v1* a1* 24*
// v1* a1* 25
// v1* a2 26
// v1* a2 27
// v2* a1* 23*
// v2* a1* 25	
// Q = v1,v2|a1|<  

// db.events.ensureIndex({venue:1, dt: 1, act: 1})	
// db.events.ensureIndex({act:1, venue: 1, dt: 1})	
// db.events.ensureIndex({dt: 1, act:1, venue: 1})	

db.events.ensureIndex({act:1, dt: 1, venue: 1})	
// a1*, 23*, v2
// a1*, 24, v1
// a1*, 25, v1
// a1*, 25, v2
// a2, 26, v1
// a2, 27, v1

// db.events.ensureIndex({dt: 1, act:1, venue: 1})
// 23*, a1*, v2*
// 24, a1*, v1*
// 25, a1*, v1*
// 25, a1*, v2*
// 26, a2, v1
// 27, a2, v1

                                   
var userVenues = ["v1", "v2"]	   
var topActs = ["a1", "a2"]		   
var now = "2011-03-20 00:00"       

// when are the top acts playing next at any of my venues?
var results = db.events.find({act: "a1", venue: {$in: userVenues}, dt: {$gte: now}})
	.sort({dt:1})
	.limit(1)
	// .toArray()
	.explain()
	
printjson(results)

results.json

{
	"cursor" : "BtreeCursor act_1_dt_1_venue_1 multi",
	"nscanned" : 1,
	"nscannedObjects" : 1,
	"n" : 1,
	"millis" : 0,
	"indexBounds" : {
		"act" : [
			[
				"a1",
				"a1"
			]
		],
		"dt" : [
			[
				"2011-03-20 00:00",
				{
					
				}
			]
		],
		"venue" : [
			[
				"v1",
				"v1"
			],
			[
				"v2",
				"v2"
			]
		]
	}
}