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If the total number of scanned items exceeds the maximum data set size limit of 1 MB, the scan stops and results are returned to the user as a LastEvaluatedKey value to continue the scan in a subsequent operation. The results also include the number of items exceeding the limit. A scan can result in no table data meeting the filter criteria.
By default, Scan operations proceed sequentially; however, for faster performance on a large table or secondary index, applications can request a parallel Scan operation by providing the Segment and TotalSegments parameters. For more information, see Parallel Scan in the Amazon DynamoDB Developer Guide.
By default, Scan uses eventually consistent reads when accessing the data in a table; therefore, the result set might not include the changes to data in the table immediately before the operation began. If you need a consistent copy of the data, as of the time that the Scan begins, you can set the ConsistentRead parameter to true.
Namespace: Amazon.DynamoDBv2
Assembly: AWSSDK.dll
Version: (assembly version)
public virtual ScanResponse Scan( ScanRequest request )
Container for the necessary parameters to execute the Scan service method.
Exception | Condition |
---|---|
InternalServerErrorException | An error occurred on the server side. |
ProvisionedThroughputExceededException | Your request rate is too high. The AWS SDKs for DynamoDB automatically retry requests that receive this exception. Your request is eventually successful, unless your retry queue is too large to finish. Reduce the frequency of requests and use exponential backoff. For more information, go to Error Retries and Exponential Backoff in the Amazon DynamoDB Developer Guide. |
ResourceNotFoundException | The operation tried to access a nonexistent table or index. The resource might not be specified correctly, or its status might not be ACTIVE. |
The following example shows how to scan items in a table.
Note: the Scan operation goes through every item in the table
to check if the item matches all the scan conditions. This makes
the Scan operation particularly slow and expensive (in terms of provisioned throughput).
We will now retrieve all items where the Pages attribute is greater
than the numerical value "200" and where the Title attribute contains
the string "Adventures".
// Create a client AmazonDynamoDBClient client = new AmazonDynamoDBClient(); // Define scan conditions Dictionary<string, Condition> conditions = new Dictionary<string, Condition>(); // Title attribute should contain the string "Adventures" Condition titleCondition = new Condition(); titleCondition.ComparisonOperator = ComparisonOperator.CONTAINS; titleCondition.AttributeValueList.Add(new AttributeValue { S = "Adventures" }); conditions["Title"] = titleCondition; // Pages attributes must be greater-than the numeric value "200" Condition pagesCondition = new Condition(); pagesCondition.ComparisonOperator = ComparisonOperator.GT;; pagesCondition.AttributeValueList.Add(new AttributeValue { N = "200" }); conditions["Pages"] = pagesCondition; // Define marker variable Dictionary<string, AttributeValue> startKey = null; do { // Create Scan request ScanRequest request = new ScanRequest { TableName = "SampleTable", ExclusiveStartKey = startKey, ScanFilter = conditions }; // Issue request ScanResult result = client.Scan(request); // View all returned items List<Dictionary<string, AttributeValue>> items = result.Items; foreach (Dictionary<string, AttributeValue> item in items) { Console.WriteLine("Item:"); foreach (var keyValuePair in item) { Console.WriteLine("{0} : S={1}, N={2}, SS=[{3}], NS=[{4}]", keyValuePair.Key, keyValuePair.Value.S, keyValuePair.Value.N, string.Join(", ", keyValuePair.Value.SS ?? new List<string>()), string.Join(", ", keyValuePair.Value.NS ?? new List<string>())); } } // Set marker variable startKey = result.LastEvaluatedKey; } while (startKey != null && startKey.Count > 0);
The following example shows how we can utilize parallel scan to partition a table into
10 segments and scan each segment in a separate thread.
To avoid resource contention between threads, the results will be written into 10 separate
files. Each segment will have a file of its own.
// Create a client AmazonDynamoDBClient client = new AmazonDynamoDBClient(); // Define scan conditions Dictionary<string, Condition> conditions = new Dictionary<string, Condition>(); // Pages attributes must be greater-than the numeric value "200" Condition pagesCondition = new Condition(); pagesCondition.ComparisonOperator = ComparisonOperator.GT; pagesCondition.AttributeValueList.Add(new AttributeValue { N = "200" }); conditions["Pages"] = pagesCondition; // Setup 10 simultaneous threads, each thread calling Scan operation // with its own segment value. int totalSegments = 10; Parallel.For(0, totalSegments, segment => { // Define marker variable Dictionary<string, AttributeValue> startKey = null; do { // Create Scan request ScanRequest request = new ScanRequest { TableName = "SampleTable", ExclusiveStartKey = startKey, ScanFilter = conditions, // Total segments to split the table into TotalSegments = totalSegments, // Current segment to scan Segment = segment }; // Issue request var result = client.Scan(request); // Write returned items to file string path = string.Format("ParallelScan-{0}-of-{1}.txt", totalSegments, segment); List<Dictionary<string, AttributeValue>> items = result.Items; using (Stream stream = File.OpenWrite(path)) using (StreamWriter writer = new StreamWriter(stream)) { foreach (Dictionary<string, AttributeValue> item in items) { writer.WriteLine("Item:"); foreach (var keyValuePair in item) { writer.WriteLine("{0} : S={1}, N={2}, SS=[{3}], NS=[{4}]", keyValuePair.Key, keyValuePair.Value.S, keyValuePair.Value.N, string.Join(", ", keyValuePair.Value.SS ?? new List<string>()), string.Join(", ", keyValuePair.Value.NS ?? new List<string>())); } } } // Set marker variable startKey = result.LastEvaluatedKey; } while (startKey != null && startKey.Count > 0); });
.NET Framework:
Supported in: 4.5, 4.0, 3.5