gistfile1.txt

// First query. Notice n = 6758. The second OR condition by definition includes the first.

> db.contacts.find(
...     {
... 
...         $or: [
...             {'foo': 32}
...             ,{'foo': 32, 'bar' : 151629}
...         ]
...     }
... ).explain();
{
	"cursor" : "BtreeCursor foo_1_bar_1",
	"nscanned" : 6758,
	"nscannedObjects" : 6758,
	"n" : 6758,
	"millis" : 7,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"isMultiKey" : false,
	"indexOnly" : false,
	"indexBounds" : {
		"foo" : [
			[
				32,
				32
			]
		],
		"bar" : [
			[
				{
					"$minElement" : 1
				},
				{
					"$maxElement" : 1
				}
			]
		]
	}
}


// Second query. Notice n = 6761.
> db.contacts.find(
...     {
... 
...         $or: [
...             {'foo': 32, 'bar' : 151629},
...             {'foo': 32}
...         ]
...     }
... ).explain(); 
{
	"clauses" : [
		{
			"cursor" : "BtreeCursor foo_1_bar_1",
			"nscanned" : 3,
			"nscannedObjects" : 3,
			"n" : 3,
			"millis" : 0,
			"nYields" : 0,
			"nChunkSkips" : 0,
			"isMultiKey" : false,
			"indexOnly" : false,
			"indexBounds" : {
				"foo" : [
					[
						32,
						32
					]
				],
				"bar" : [
					[
						151629,
						151629
					]
				]
			}
		},
		{
			"cursor" : "BtreeCursor foo_1_bar_1",
			"nscanned" : 6758,
			"nscannedObjects" : 6758,
			"n" : 6755,
			"millis" : 33,
			"nYields" : 0,
			"nChunkSkips" : 0,
			"isMultiKey" : false,
			"indexOnly" : false,
			"indexBounds" : {
				"foo" : [
					[
						32,
						32
					]
				],
				"bar" : [
					[
						{
							"$minElement" : 1
						},
						{
							"$maxElement" : 1
						}
					]
				]
			}
		}
	],
	"nscanned" : 6761,
	"nscannedObjects" : 6761,
	"n" : 6758,
	"millis" : 33
}