Tablestore:Read data

Query methods

Tablestore provides the GetRow, BatchGetRow, and GetRange operations to allow you to read data. Before you read data, select the appropriate query method based on the actual query scenario.

Prerequisites

• The OTSClient instance is initialized. For more information, see Initialize an OTSClient instance.
• A data table is created. Data is written to the table.

Read a single row of data

You can call the GetRow operation to read a single row of data. After you call the GetRow operation, one of the following results may be returned:

• If the row exists, the primary key columns and attribute columns of the row are returned.

• If the row does not exist, no row is returned and no error is reported.

API operation

/// <summary>
/// Read a single row of data based on the specified primary key information. 
/// </summary>
/// <param name="request">Data query request</param>
/// <returns>Response of GetRow</returns>
public GetRowResponse GetRow(GetRowRequest request);

/// <summary>
/// The asynchronous mode of GetRow. 
/// </summary>
public Task<GetRowResponse> GetRowAsync(GetRowRequest request);          

Parameters

Sample code

    // Specify the primary key information of the row. The primary key information must be the same as the primary key information that is specified in TableMeta when the table is created. 
    PrimaryKey primaryKey = new PrimaryKey();
    primaryKey.Add("pk0", new ColumnValue(0));
    primaryKey.Add("pk1", new ColumnValue("abc"));

    try
    {
        // Construct a query request object. The entire row is read if no column is specified. 
        var request = new GetRowRequest(TableName, primaryKey);

        // Call the GetRow operation to query data. 
        var response = otsClient.GetRow(request);

        // Return the data of the row. In this example, the sample code that is used to return the data of the row is omitted. To view the detailed sample code, visit the GitHub link that is provided for this sample code. 

        // If the operation is successful, no exception is returned. 
        Console.WriteLine("Get row succeeded.");
    }
    catch (Exception ex)
    {
        // If the operation fails, an exception is returned. Handle the exception. 
        Console.WriteLine("Update table failed, exception:{0}", ex.Message);
    }
            

    // Specify the primary key information of the row. The primary key information must be the same as the primary key information that is specified in TableMeta when the table is created. 
    PrimaryKey primaryKey = new PrimaryKey();
    primaryKey.Add("pk0", new ColumnValue(0));
    primaryKey.Add("pk1", new ColumnValue("abc"));

    var rowQueryCriteria = new SingleRowQueryCriteria("SampleTable");
    rowQueryCriteria.RowPrimaryKey = primaryKey;

    // Condition 1: The value of the col0 column is 5. 
    var filter1 = new RelationalCondition("col0",
                RelationalCondition.CompareOperator.EQUAL,
                new ColumnValue(5));

    // Condition 2: The value of the col1 column is not ff. 
    var filter2 = new RelationalCondition("col1", RelationalCondition.CompareOperator.NOT_EQUAL, new ColumnValue("ff"));

    // Construct a combination of Condition 1 and Condition 2. The conditions are evaluated by using the OR operator. 
    var filter = new CompositeCondition(CompositeCondition.LogicOperator.OR);
    filter.AddCondition(filter1);
    filter.AddCondition(filter2);

    rowQueryCriteria.Filter = filter;

    // Specify col0 and col1 as the columns that you want to read. Tablestore queries the values of the col0 and col1 columns, and then returns the rows that meet the filter conditions. 
    rowQueryCriteria.AddColumnsToGet("col0");
    rowQueryCriteria.AddColumnsToGet("col1");

    // Construct a GetRowRequest object. 
    var request = new GetRowRequest(rowQueryCriteria);

    try
    {
        // Perform the query. 
        var response = otsClient.GetRow(request);

        // Return data or perform the related logical operation. In this example, the code that is used to return data or perform the related logical operation is omitted. 

        // If the operation is successful, no exception is returned. 
        Console.WriteLine("Get row with filter succeeded.");
    }
    catch (Exception ex)
    {
        // If the operation fails, an exception is returned. Handle the exception. 
        Console.WriteLine("Get row with filter failed, exception:{0}", ex.Message);
    }          

Read multiple rows of data at a time

You can call the BatchGetRow operation to read multiple rows of data from one or more tables at a time. The BatchGetRow operation consists of multiple GetRow operations. When you call the BatchGetRow operation, the process of constructing each GetRow operation is the same as the process of constructing the GetRow operation when you call the GetRow operation.

If you call the BatchGetRow operation, each GetRow operation is separately performed, and Tablestore separately returns the response to each GetRow operation.

Usage notes

• When you call the BatchGetRow operation to read multiple rows at a time, some rows may fail to be read. If this happens, Tablestore does not return exceptions, but returns BatchGetRowResponse in which the information about the failed rows are included. Therefore, when you call the BatchGetRow operation, you must check the return values to determine whether data is successfully read from each row.

• The BatchGetRow operation uses the same parameter settings for all rows. For example, if the ColumnsToGet parameter is set to [colA], only the value of the colA column is read from all rows.

• You can call the BatchGetRow operation to read a maximum of 100 rows at a time.

API operation

/// <summary>
/// <para>Read multiple rows of data from one or more tables at a time. </para>
/// <para>The BatchGetRow operation is a set of GetRow operations. Each operation is performed, returns results, and computes the consumed capacity units (CUs) in an independent way. </para>
/// Compared with a large number of GetRow operations, the BatchGetRow operation can reduce the response time and increase the data read rate. 
/// </summary>
/// <param name="request">Request instance</param>
/// <returns>Response instance</returns>
public BatchGetRowResponse BatchGetRow(BatchGetRowRequest request);

/// <summary>
/// The asynchronous mode of BatchGetRow. 
/// </summary>
public Task<BatchGetRowResponse> BatchGetRowAsync(BatchGetRowRequest request);            

Sample code

The following sample code provides an example on how to read 10 rows of data at a time:

// Construct a request object to read multiple rows of data at a time. Specify the primary key information for 10 rows of data. 
List<PrimaryKey> primaryKeys = new List<PrimaryKey>();
for (int i = 0; i < 10; i++)
{
    PrimaryKey primaryKey = new PrimaryKey();
    primaryKey.Add("pk0", new ColumnValue(i));
    primaryKey.Add("pk1", new ColumnValue("abc"));
    primaryKeys.Add(primaryKey);
}

try
{
    BatchGetRowRequest request = new BatchGetRowRequest();
    request.Add(TableName, primaryKeys);

    // Call the BatchGetRow operation to read 10 rows of data. 
    var response = otsClient.BatchGetRow(request);
    var tableRows = response.RowDataGroupByTable;
    var rows = tableRows[TableName];

    // Return the data of the rows. In this example, the sample code that is used to return the data of the rows is omitted. To view the detailed sample code, visit the GitHub link that is provided for this sample code. 

    // When you call the BatchGetRow operation to read multiple rows at a time, some rows may fail to be read. You must check the return values to determine whether data is successfully read from each row. To view the detailed sample code, visit the GitHub link that is provided for this sample code. 
}
catch (Exception ex)
{
    // If the operation fails, an exception is returned. Handle the exception. 
    Console.WriteLine("Batch get row failed, exception:{0}", ex.Message);
}            

To view the detailed sample code, visit BatchGetRow@GitHub.

Read data whose primary key values are within a specific range

You can call the GetRange operation to read data whose primary key values are in the specified range.

The GetRange operation allows you to read data whose primary key values are in the specified range in a forward or backward direction. You can also specify the number of rows to read. If the range is large and the number of scanned rows or the volume of scanned data exceeds the upper limit, the scan stops, and the rows that are read and information about the primary key of the next row are returned. You can initiate a request to start from where the last operation left off and read the remaining rows based on the information about the primary key of the next row returned by the previous operation.

Usage notes

The GetRange operation follows the leftmost matching principle. Tablestore compares values in sequence from the first primary key column to the last primary key column to read data whose primary key values are in the specified range. For example, the primary key of a data table consists of the following primary key columns: PK1, PK2, and PK3. When data is read, Tablestore first determines whether the PK1 value of a row is in the range that is specified for the first primary key column. If the PK1 value of a row is in the range, Tablestore stops determining whether the values of other primary key columns of the row are in the ranges that are specified for each primary key column and returns the row. If the PK1 value of a row is not in the range, Tablestore continues to determine whether the values of other primary key columns of the row are in the ranges that are specified for each primary key column in the same manner as PK1.

If one of the following conditions is met, the GetRange operation may stop and return data:

• The amount of scanned data reaches 4 MB.

• The number of scanned rows reaches 5,000.

• The number of returned rows reaches the upper limit.

• The read throughput is insufficient to read the next row of data because all reserved read throughput is consumed.

API operation

/// <summary>
/// Query data whose primary key values are in the specified range. 
/// </summary>
/// <param name="request">Request instance</param>
/// <returns>Response instance</returns>
public GetRangeResponse GetRange(GetRangeRequest request);

/// <summary>
/// The asynchronous mode of GetRange. 
/// </summary>
/// <param name="request"></param>
/// <returns></returns>
public Task<GetRangeResponse> GetRangeAsync(GetRangeRequest request);              

Parameters

Sample code

The following sample code provides an example on how to read data whose primary key values are in the specified range:

// Read all rows whose primary key values are in the range of (0, INF_MIN) to (100, INF_MAX). 
var inclusiveStartPrimaryKey = new PrimaryKey();
inclusiveStartPrimaryKey.Add("pk0", new ColumnValue(0));
inclusiveStartPrimaryKey.Add("pk1", ColumnValue.INF_MIN);

var exclusiveEndPrimaryKey = new PrimaryKey();
exclusiveEndPrimaryKey.Add("pk0", new ColumnValue(100));
exclusiveEndPrimaryKey.Add("pk1", ColumnValue.INF_MAX);

try
{
    // Construct a request object to read data whose primary key values are in the specified range. 
    var request = new GetRangeRequest(TableName, GetRangeDirection.Forward,
                    inclusiveStartPrimaryKey, exclusiveEndPrimaryKey);

    var response = otsClient.GetRange(request);

    // Continue the read operation if only part of data is returned. 
    var rows = response.RowDataList;
    var nextStartPrimaryKey = response.NextPrimaryKey;
    while (nextStartPrimaryKey != null)
    {
        request = new GetRangeRequest(TableName, GetRangeDirection.Forward,
                        nextStartPrimaryKey, exclusiveEndPrimaryKey);
        response = otsClient.GetRange(request);
        nextStartPrimaryKey = response.NextPrimaryKey;
        foreach (RowDataFromGetRange row in response.RowDataList)
        {
            rows.Add(row);
        }
    }

    // Return the data of the rows. In this example, the sample code that is used to return the data of the rows is omitted. To view the detailed sample code, visit the GitHub link that is provided for this sample code. 

    // If the operation is successful, no exception is returned. 
    Console.WriteLine("Get range succeeded");
}
catch (Exception ex)
{
    // If the operation fails, an exception is returned. Handle the exception. 
    Console.WriteLine("Get range failed, exception:{0}", ex.Message);
}            

To view the detailed sample code, visit GetRange@GitHub.

Read data whose primary key values are within a specific range by using an iterator

You can call the GetRangeIterator operation to read data whose primary key values are in the specified range by using an iterator.

API operation

/// <summary>
/// Obtain data from multiple rows whose primary key values are within the specified range. Return the iterator used to process each row of data. 
/// </summary>
/// <param name="request"><see cref="GetIteratorRequest"/></param>
/// <returns>Return the <see cref="RowDataFromGetRange"/> iterator. </returns>
public IEnumerable<RowDataFromGetRange> GetRangeIterator(GetIteratorRequest request);           

Sample code

The following sample code provides an example on how to read all rows whose primary key values are in the range of (0, "a") to (1000, "xyz"):

// Read all rows whose primary key values are in the range of (0, "a") to (1000, "xyz"). 
PrimaryKey inclusiveStartPrimaryKey = new PrimaryKey();
inclusiveStartPrimaryKey.Add("pk0", new ColumnValue(0));
inclusiveStartPrimaryKey.Add("pk1", new ColumnValue("a"));

PrimaryKey exclusiveEndPrimaryKey = new PrimaryKey();
exclusiveEndPrimaryKey.Add("pk0", new ColumnValue(1000));
exclusiveEndPrimaryKey.Add("pk1", new ColumnValue("xyz"));

// Construct a CapacityUnit object to record the number of CUs consumed by iteration. 
var cu = new CapacityUnit(0, 0);

try
{
    // Construct a GetIteratorRequest object. Filter conditions are supported. 
    var request = new GetIteratorRequest(TableName, GetRangeDirection.Forward, inclusiveStartPrimaryKey,
                                                exclusiveEndPrimaryKey, cu);

    var iterator = otsClient.GetRangeIterator(request);
    // Use the iterator that reads data in a traversal way. 
    foreach (var row in iterator)
    {
        // Execute the processing logic. 
    }

    Console.WriteLine("Iterate row succeeded");
} 
catch (Exception ex)
{
    Console.WriteLine("Iterate row failed, exception:{0}", ex.Message);
}            

To view the detailed sample code, visit GetRangeIterator@GitHub.