Query Broadcast when query contains a subset of shard key

[Avinash Vyas]

Hi 

    I am trying to understand the behavior of the query router in a sharded environment w.r.t. to a shard key. I have a collection with 1 million documents of following types

   { 

     _id: <string>, 

     y:1, 

     data:string

   } 

    where value of _id is unique in every document but is chosen by me. 

    In my sharded cluster, I have used _id as my shard key. My understanding is _id is always unique but the sh.status() shows the following 

               test.mycollection

                        shard key: { "_id" : 1 }

                        unique: false

                        balancing: true

                        chunks:

                                wshard1 178

                                wshard2 178

                                wshard3 176

                                wshard4 177

    I have some questions:

    a) Why is the unique: false when for the _id index we know that its is true by default.

    b) Does it matter if the unique is false or true, Shouldn't all the documents with a particular value for the shard key be stored on the same shard.

    c) When I issue a query (or a conditional update with query) { _id : "string1", y:100} the query (query from conditional update) is broadcast to all the shards. Given that my shard key is _id (which is unique), a document with a specific _id can only reside on one shard and why can't the query router identify the correct shard and send it as a target operation. 

    d) More generally when all the documents for a shard key x:"string1" will be stored in one shard, any query such as {x: "string1", y:10} should be a targeted operation rather than a broadcast operation.

Thanks

Avinash

[Avinash Vyas]

The situation I explained below is not completely correct. The phenomenon we see in our application cannot be explained with this example. I will update the example once we have nailed down the query that is broadcasted. 

[Asya Kamsky]

If you look at indexes on the collection in question via
test.mycollection.getIndexes() you will notice that the _id index
never shows that it is unique.

Nevertheless it is always unique.

Your second message indicates what I suspected when reading your
description. Something else must be going on - queries and updates
which specify equality on the shard key will always be targeted to a
single shard only (the one that all documents with this particular
value of the shard key live).

Asya

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