Sharding: Indexing

  • Indexing a sharded database is performed locally, per shard.
    There is no multi-shard indexing process.
  • Indexes use the same syntax in sharded and non-sharded databases.
  • Most indexing features supported by non-sharded databases are also supported by sharded databases. Unsupported features are listed below.

  • In this page:


Indexing each database shard is basically similar to indexing a non-sharded database.
As each shard holds and manages a unique dataset, indexing is performed per-shard and indexes are stored only on the shard that created and uses them.

Map-Reduce Indexes on a Sharded Database

Map-reduce indexes on a sharded database are used to reduce data both over each shard during indexation, and on the orchestrator machine each time a query uses them.

  1. Reduction by each shard during indexation
    Similarly to non-sharded databases, when shards index their data they reduce the results by map-reduce indexes.
  2. Reduction by the orchestrator during queries
    When a query is executed over map-reduce indexes the orchestrator distributes the query to the shards, collects and combines the results, and then reduces them again.

Learn about querying map-reduce indexes in a sharded database here.

Unsupported Indexing Features

Unsupported or yet-unimplemented indexing features include:

  • Rolling index deploymeny
    Rolling index deploymeny is not supported in a Sharded Database.
  • Loading documents from other shards
    Loading a document during indexing is possible only if the document resides on the shard.
    Consider the below index, for example, that attempts to load a document.
    If the requested document is stored on a different shard, the load operation will be ignored.
    Map = products => from product in products
                              select new Result
                                  CategoryName = LoadDocument<Category>(product.Category).Name

    You can make sure that documents share a bucket, and can therefore locate and load each other, using the $ syntax.

  • Map-Reduce Output Documents
    Using OutputReduceToCollection to output the results of a map-reduce index to a collection is not supported in a Sharded Database.
  • Custom Sorters are not supported in a Sharded Database.