Working with Global Secondary Indexes: .NET

You can use the Amazon SDK for .NET low-level API to create an Amazon DynamoDB table with one or more global secondary indexes, describe the indexes on the table, and perform queries using the indexes. These operations map to the corresponding DynamoDB operations. For more information, see the Amazon DynamoDB API Reference.

The following are the common steps for table operations using the .NET low-level API.

Create a table with a Global Secondary Index

You can create global secondary indexes at the same time that you create a table. To do this, use CreateTable and provide your specifications for one or more global secondary indexes. The following C# code example creates a table to hold information about weather data. The partition key is Location and the sort key is Date. A global secondary index named PrecipIndex allows fast access to precipitation data for various locations.

The following are the steps to create a table with a global secondary index, using the .NET low-level API.

The following C# code example demonstrates the preceding steps. The code creates a table (WeatherData) with a global secondary index (PrecipIndex). The index partition key is Date and its sort key is Precipitation. All of the table attributes are projected into the index. Users can query this index to obtain weather data for a particular date, optionally sorting the data by precipitation amount.

Because Precipitation is not a key attribute for the table, it is not required. However, WeatherData items without Precipitation do not appear in PrecipIndex.

client = new AmazonDynamoDBClient();
string tableName = "WeatherData";

// Attribute definitions
var attributeDefinitions = new List<AttributeDefinition>()
{
    {new AttributeDefinition{
        AttributeName = "Location",
        AttributeType = "S"}},
    {new AttributeDefinition{
        AttributeName = "Date",
        AttributeType = "S"}},
    {new AttributeDefinition(){
        AttributeName = "Precipitation",
        AttributeType = "N"}
    }
};

// Table key schema
var tableKeySchema = new List<KeySchemaElement>()
{
    {new KeySchemaElement {
        AttributeName = "Location",
        KeyType = "HASH"}},  //Partition key
    {new KeySchemaElement {
        AttributeName = "Date",
        KeyType = "RANGE"}  //Sort key
    }
};

// PrecipIndex
var precipIndex = new GlobalSecondaryIndex
{
    IndexName = "PrecipIndex",
    ProvisionedThroughput = new ProvisionedThroughput
    {
        ReadCapacityUnits = (long)10,
        WriteCapacityUnits = (long)1
    },
    Projection = new Projection { ProjectionType = "ALL" }
};

var indexKeySchema = new List<KeySchemaElement> {
    {new KeySchemaElement { AttributeName = "Date", KeyType = "HASH"}},  //Partition key
    {new KeySchemaElement{AttributeName = "Precipitation",KeyType = "RANGE"}}  //Sort key
};

precipIndex.KeySchema = indexKeySchema;

CreateTableRequest createTableRequest = new CreateTableRequest
{
    TableName = tableName,
    ProvisionedThroughput = new ProvisionedThroughput
    {
        ReadCapacityUnits = (long)5,
        WriteCapacityUnits = (long)1
    },
    AttributeDefinitions = attributeDefinitions,
    KeySchema = tableKeySchema,
    GlobalSecondaryIndexes = { precipIndex }
};

CreateTableResponse response = client.CreateTable(createTableRequest);
Console.WriteLine(response.CreateTableResult.TableDescription.TableName);
Console.WriteLine(response.CreateTableResult.TableDescription.TableStatus);

You must wait until DynamoDB creates the table and sets the table status to ACTIVE. After that, you can begin putting data items into the table.

Describe a table with a Global Secondary Index

To get information about global secondary indexes on a table, use DescribeTable. For each index, you can access its name, key schema, and projected attributes.

The following are the steps to access global secondary index information for a table using the .NET low-level API.

The following C# code example demonstrates the preceding steps.

client = new AmazonDynamoDBClient();
string tableName = "WeatherData";

DescribeTableResponse response = client.DescribeTable(new DescribeTableRequest { TableName = tableName});

List<GlobalSecondaryIndexDescription> globalSecondaryIndexes =
response.DescribeTableResult.Table.GlobalSecondaryIndexes;

// This code snippet will work for multiple indexes, even though
// there is only one index in this example.

foreach (GlobalSecondaryIndexDescription gsiDescription in globalSecondaryIndexes) {
     Console.WriteLine("Info for index " + gsiDescription.IndexName + ":");

     foreach (KeySchemaElement kse in gsiDescription.KeySchema) {
          Console.WriteLine("\t" + kse.AttributeName + ": key type is " + kse.KeyType);
     }

      Projection projection = gsiDescription.Projection;
      Console.WriteLine("\tThe projection type is: " + projection.ProjectionType);

      if (projection.ProjectionType.ToString().Equals("INCLUDE")) {
           Console.WriteLine("\t\tThe non-key projected attributes are: "
                + projection.NonKeyAttributes);
      }
}

Query a Global Secondary Index

You can use Query on a global secondary index, in much the same way you Query a table. You need to specify the index name, the query criteria for the index partition key and sort key (if present), and the attributes that you want to return. In this example, the index is PrecipIndex, which has a partition key of Date and a sort key of Precipitation. The index query returns all of the weather data for a particular date, where the precipitation is greater than zero.

The following are the steps to query a global secondary index using the .NET low-level API.

The attribute name Date is a DynamoDB reserved word. Therefore, you must use an expression attribute name as a placeholder in the KeyConditionExpression.

The following C# code example demonstrates the preceding steps.

client = new AmazonDynamoDBClient();

QueryRequest queryRequest = new QueryRequest
{
    TableName = "WeatherData",
    IndexName = "PrecipIndex",
    KeyConditionExpression = "#dt = :v_date and Precipitation > :v_precip",
    ExpressionAttributeNames = new Dictionary<String, String> {
        {"#dt", "Date"}
    },
    ExpressionAttributeValues = new Dictionary<string, AttributeValue> {
        {":v_date", new AttributeValue { S =  "2013-08-01" }},
        {":v_precip", new AttributeValue { N =  "0" }}
    },
    ScanIndexForward = true
};

var result = client.Query(queryRequest);

var items = result.Items;
foreach (var currentItem in items)
{
    foreach (string attr in currentItem.Keys)
    {
        Console.Write(attr + "---> ");
        if (attr == "Precipitation")
        {
            Console.WriteLine(currentItem[attr].N);
    }
    else
    {
        Console.WriteLine(currentItem[attr].S);
    }

         }
     Console.WriteLine();
}