lesson1

Unit 2, Lesson 1 - Getting started with indexes

RavenDB Does No Table Scans!

What does it mean? In Unit-1 you wrote a query similar to the following:

var orders = (
    from order in session.Query<Order>()
    where order.Company == "companies/1-A"
    orderby order.OrderedAt
    select order
    ).ToList();

Remembering RQL syntax:

from Orders
where Company = "companies/1-A"
order by OrderedAt

You might assume that the following pseudo code is run in the server side. Am I right?

//..
var results = new List<Order>();

foreach (var o in GetDocumentsFor("Orders"))
{
    if (o.Company == "companies/1-A")
        results.Add(o);
}

var orderedResults = results.Sort((a,b) => a.OrderedAt.CompareTo(b.OrderedAt));
// ..

The previous code is known as table scan (tables?!). That is, all documents are analyzed and those that meet the predicate are selected. This is quite efficient, as long as the number of items you have in the database is small. But it fails pretty horribly the moment your data size reaches any significant size.

RavenDB Uses Indexes to Speed Up the Queries

Even when you do not create an index, RavenDB will use one to execute queries. In fact there are no O(N) operations in general in RavenDB queries. Using indexes, queries in RavenDB are O(logN) operations. For those who don't care about algorithms complexity analysis, the difference is between waiting 30 minutes for a result and getting it right away.

RavenDB is safe by default and whenever you make a query, the query optimizer will try to select an appropriate index to use. If there is no such appropriate index, then the query optimizer will create an index for you.

Exercise 1: Confirming that index can be automatically created

In this lesson you will confirm that RavenDB creates indexes automatically when it is needed.

Step 1: Access Northwind database using the Management Studio

Start RavenDB console (if you didn't do it yet), and using the web browser, access the RavenDB Management Studio at the address http://localhost:8080 (which is the default address. Change it if you need). Then open the Northwind database which you created in the previous unit (Lesson 1).

Step 2: Delete all the existing indexes

In the left panel, select the Indexes section. Then, select all indexes and then click on the Delete button.

Step 3: Create and execute a program which will perform a query

We could do the following query directly from RavenDB Management Studio.

from Orders
where Company = "companies/1-A"
order by OrderedAt

Let's make it more interesting.

Create a new Console Application, add the proper NuGet RavenDB reference (RavenDB.Client), then change the Program.cs content to the following:

using System;
using System.Linq;
using Raven.Client.Documents;

namespace IndexingSample
{
    class Program
    {
        static void Main()
        {
            using (var session = DocumentStoreHolder.Store.OpenSession())
            {
                var ordersIds = (
                    from order in session.Query<Order>()
                    where order.Company == "companies/1-A"
                    orderby order.OrderedAt
                    select order.Id
                    ).ToList();

                foreach (var id in ordersIds)
                {
                    Console.WriteLine(id);
                }
            }
        }
    }

    public static class DocumentStoreHolder
    {
        private static readonly Lazy<IDocumentStore> LazyStore =
            new Lazy<IDocumentStore>(() =>
            {
                var store = new DocumentStore
                {
                    Urls = new[] { "http://localhost:8080" },
                    Database = "Northwind"
                };

                return store.Initialize();
            });

        public static IDocumentStore Store =>
            LazyStore.Value;
    }

    public class Order
    {
        public string Id { get; set; }
        public string Company { get; set; }
        public DateTimeOffset OrderedAt { get; set; }
    }
}

Note that the Order model class has just enough code here. By default, Raven.Client will preserve old properties even if they are not in the model. This behavior can be changed via the PreserveDocumentPropertiesNotFoundOnModel convention

Step 4: Run the program

Step 5: Confirm that a new index was automatically created

Return to the Management Studio and go to the Indexes/List of Indexes section. Surprise! A new index named Auto/Orders/ByCompanyAndOrderedAt was created for you.

RavenDB chooses good names for "auto created indexes".

Great Job!

Let's move onto Lesson 2 and learn how to create and index on your own.