如何通过mgo（golang）创建哈希索引

Question

How can I create (or ensure) hashed index with mgo package?

I need a go code to be equivalent with this:

>> db.collection.createIndex( { _id: "hashed" } )

I have tried using runCommand but there is only ‍createIndexes command that want a list of index specs. And I have no idea what is that and how I can create index specs.

Answer 1

You can do it as documented at Collection.EnsureIndex:

Other kinds of indexes are also supported through that API. Here is an example:

index := Index{
    Key: []string{"$2d:loc"},
    Bits: 26,
}
err := collection.EnsureIndex(index)

The example above requests the creation of a "2d" index for the "loc" field.

So basically, you have the format $<indexType>:<indexedField>, as shown below:

package main

import mgo "gopkg.in/mgo.v2"

const (
    db   = "so_hashed_idx"
    coll = "testcoll"
)

func main() {
    var s *mgo.Session
    var err error

    if s, err = mgo.Dial("127.0.0.1:27017"); err != nil {
        panic(err)
    }

    // An index spec is nothing more than a fancy word for the keys
    // or the key/value pairs handed over to the Key slice of the
    // Index type.
    idx := mgo.Index{
        Key: []string{"$hashed:_id"},
    }

    if err := s.DB(db).C(coll).EnsureIndex(idx); err != nil {
        panic(err)
    }
}

Building and running the above results in so_hashed_idx.testcoll showing its indices as follows

> db.testcoll.getIndices()
[
    {
        "v" : 1,
        "key" : {
            "_id" : 1
        },
        "name" : "_id_",
        "ns" : "so_hashed_idx.testcoll"
    },
    {
        "v" : 1,
        "key" : {
            "_id" : "hashed"
        },
        "name" : "_id_hashed",
        "ns" : "so_hashed_idx.testcoll"
    }
]