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mongodbsharding

Mongodb Hashed Sharding

If I choose {a:1,b:1,c:1} as my shard key and in my query I filter {a:1} in a hashed sharding strategy , is the query a targeted operation or it is broadcasting to every shard in the cluster?

If it is targeted operation how mongodb determine it? as hash of {a:1} is completely differ from hash of {a:1,b:1,c:1}

Solution

  • The simple answear is: Yes.

    Look at it this way:

    Let's assume you have got the following collection:

        //1
    {
      a: 1,
      b: 1,
      c: 1,
      d: 1

    },
    //2
    {
      a: 1,
      b: 1,
      c: 1,
      d: 2
    },
    //3
    {
      a: 1,
      b: 1,
      c: 2,
      d: 5
    }

    According to your index, docs 1 and 2 must be at the same bulk (let's say, on shard number 1) while doc 3 could be stored on a different bulk (let's say, on shard number 2)

    Now, if you search for {a: 1}, all three docs should appear. Meaning that mongo had to distribute the que both to shard no.1 and shard no.2.

    As for your second question, in MongoDb, you cannot perform Compound-Hashed-Index at all (and even if you could, than... yes. The hashed value would probably be diff)