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What are Indexes?

Indexes are server-side functions that define which fields and what values documents can be searched on. They are the only way to satisfy queries in RavenDB. The whole indexing process is done in the background and is triggered whenever data is added or changed. This approach allows the server to respond quickly even when large amounts of data have changed and avoids costly table scan operations. A consequence of this choice is that the results might be stale. You can read more about this (including mitigation techniques) on the Stale Indexes page.

The core of every index is its mapping function that utilizes LINQ-like syntax, and the result of such a mapping is converted to a Lucene index entry which is persisted for future use to avoid re-indexing each time the query is issued and keep response times minimal.

Basic Example

In our example, we will create an index that will map documents from the Employees collection and enable querying by FirstName, LastName, or both.

  • First we need to create an index. One way to create it is to use the AbstractIndexCreationTask, but there are other ways available as well (you can read about them here).

public class Employees_ByFirstAndLastName : AbstractIndexCreationTask<Employee>
{
    public Employees_ByFirstAndLastName()
    {
        Map = employees => from employee in employees
                           select new
                           {
                               FirstName = employee.FirstName,
                               LastName = employee.LastName
                           };
    }
}

  • The next step is to send an index to a server (more about index deployment options can be found here) so the indexing process can start indexing documents.

// save index on server
new Employees_ByFirstAndLastName().Execute(store);

  • Now our index can be queried and indexed results can be returned.

IList<Employee> results = session
    .Query<Employee, Employees_ByFirstAndLastName>()
    .Where(x => x.FirstName == "Robert")
    .ToList();

More examples with detailed descriptions can be found here.

Remarks

Remember

A frequent mistake is to treat indexes as SQL Views, but they are not analogous. The result of a query for the given index is a full document, not only the fields that were indexed.

This behavior can be altered by storing fields and doing projections.