mongodb - 在单个数组字段上创建复合索引

Question

我想在将保存数组的字段上创建一个索引。默认情况下，当在这样的字段上创建索引时，mongo 会分别索引每个数组中的每个项目。

但是，我希望索引改为复合索引，类似于子文档字段上的索引（我希望索引是唯一的，并且['a', 'b']与 不同['b', 'a']）。有没有办法在mongo中做到这一点？

Answer 1

You can do this by storing the data as follows:

{ 
    field: { v: [ a, "b" ] }
}

And then do an index on:

db.collection.ensureIndex( { field: 1 } );

I know it's a bit counter intuitive, but an index on "field" will now use as index value:

v: [ a, "b" ]

And not each a and "b" individually. Of course you can use something else for "v", but it is important that the value for field is a document, and not an array.

A query on:

db.collection.find( { field: { v: [ 'a', 'b' ] } } )

Will then happily use the index:

{
    "cursor" : "BtreeCursor field_1",
    "isMultiKey" : false,
    "n" : 1,
    "nscannedObjects" : 1,
    "nscanned" : 1,
    "nscannedObjectsAllPlans" : 1,
    "nscannedAllPlans" : 1,
    "scanAndOrder" : false,
    "indexOnly" : false,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "millis" : 0,
    "indexBounds" : {
        "field" : [
            [
                {
                    "v" : [ "a", "b" ]
                },
                {
                    "v" : [ "a", "b" ]
                }
            ]
        ]
    },
    "server" : "whisky:27017"
}

Answer 2

对于在包装字段上索引时它是否使用索引的问题，是的，它确实使用了索引。以下来自 shell 的语句证明了这一点。

> db.arrayindtest.insert({_id:1, f:{a:["a","b"]}})
> db.arrayindtest.insert({_id:2, f:{a:["b","a"]}})
> db.arrayindtest.insert({_id:3, f:{a:["a","b", "c"]}})
> db.arrayindtest.ensureIndex({f:1})
> db.arrayindtest.find({f:{a:["a","b"]}})
{ "_id" : 1, "f" : { "a" : [  "a",  "b" ] } }
> db.arrayindtest.find({f:{a:["a","b"]}}).explain()
{
        "cursor" : "BtreeCursor f_1",
        "isMultiKey" : false,
        "n" : 1,
        "nscannedObjects" : 1,
        "nscanned" : 1,
        "nscannedObjectsAllPlans" : 1,
        "nscannedAllPlans" : 1,
        "scanAndOrder" : false,
        "indexOnly" : false,
        "nYields" : 0,
        "nChunkSkips" : 0,
        "millis" : 0,
        "indexBounds" : {
                "f" : [
                        [
                                {
                                        "a" : [
                                                "a",
                                                "b"
                                        ]
                                },
                                {
                                        "a" : [
                                                "a",
                                                "b"
                                        ]
                                }
                        ]
                ]
        },
        "server" : "sridhar-PC:27017"
}
>