How to output multiple files dynamically using U-SQL custom Extractors?

Question

I have created a U-SQL query which reads the data from multiple files. Now I have to output the data in multiple files in some other folder. I have the below example,

U-SQL Output in Azure Data Lake

Used the below script,

        DECLARE @storagePath string = @"E:\";
        DECLARE @inputFileSetName string = @"dbo.file{*}.tsv"; 
        DECLARE @outputFileName string = @"dbo.files.csv";

        DECLARE @input string = String.Concat(@storagePath, 
        @inputFileSetName);
        DECLARE @output = string.Concat(@storagePath, @outputFileName);
        @searchlog =
        EXTRACT Id string,
        Name string,
        Address string

        FROM @input
        USING new USQLApplication3.SampleExtractor(Encoding.UTF8);
        @transactions =
        SELECT *,
        ROW_NUMBER() OVER(PARTITION BY Id ORDER BY Id DESC) AS 
        RowNumber_12345
        FROM @searchlog;

       @result =
       SELECT EXTRACT Id,
        Name,
        Address 
       FROM @transactions
       WHERE RowNumber_12345 == 1;

       OUTPUT @result 
       TO @output
       USING Outputters.Csv(encoding : Encoding.UTF8, quoting : false);

Below is the C# code for custom Extractors,

public class SampleExtractor : IExtractor
{
    private Encoding _encoding;
    private byte[] _row_delim;
    private char _col_delim;

    public SampleExtractor(Encoding encoding, string row_delim = "\n", char col_delim = '\t')
    {
        this._encoding = ((encoding == null) ? Encoding.UTF8 : encoding);
        this._row_delim = this._encoding.GetBytes(row_delim);
        this._col_delim = col_delim;
    }

    public override IEnumerable<IRow> Extract(IUnstructuredReader input, IUpdatableRow output)
    {
        string line;
        //Read the input line by line
        foreach (Stream current in input.Split(_encoding.GetBytes("\n")))
        {
            using (StreamReader streamReader = new StreamReader(current, this._encoding))
            {
                line = streamReader.ReadToEnd().Trim();
                //Split the input by the column delimiter
                string[] parts = line.Split(this._col_delim);
                foreach (string part in parts)
                {
                    string res = part;
                    if (res != null)
                    {
                        res = "\"" + res.Replace("\"", "\"\"") + "\"";
                    }
                    output.Set<string>(count, res);
                }

            }
            yield return output.AsReadOnly();

        }
       yield break;
    }
}

How can I dynamically create multiple files on output? As of now, it is creating only a single file.

Edited:

I have also tried with the multiple output in a single u-sql script,

      @x = SELECT * FROM (VALUES( "A", 10, 20), ("A", 11, 21), ("B", 10, 30), ("B", 100, 200)) AS T(name, value1, value2);

     // Generate the script to do partitioned output based on name column:
     DECLARE @out string ="E:/genscript.usql";
     @stmts = 
         SELECT "@res=SELECT value1, value2 FROM @x WHERE name == 
         \""+name+"\"; 
     OUTPUT 
          @res TO \"E:/"+name+".csv\" USING Outputters.Csv();" AS output 
          FROM (SELECT DISTINCT name FROM @x) AS x;

     OUTPUT @stmts TO @out
           USING Outputters.Text(delimiter:' ', quoting:false);

     @path = EXTRACT path string FROM @out USING Extractors.Text() ;
     OUTPUT @path  TO "E:/{*}.usql" 
           USING Outputters.Text(delimiter:' ', quoting:false);

But it is showing the following error,

Severity Code Description Project File Line Suppression State Error E_CSC_USER_READFROMPREVIOUSOUTPUT: The EXTRACT statement reads/references from location E:\genscript.usql that is the target of a previous OUTPUT statement. Description: Compiler does not support OUTPUT to and then reading/referencing from the same file in a single job. Resolution: Please make one job to OUTPUT to the file and a different job to read/reference the same file.

What should I do now to output multiple files from a single u-sql or how I access another u-sql script file from current u-sql script?

Answer 1

You can simply use multiple OUTPUT commands, for example

OUTPUT @result1
TO @output1
USING Outputters.Csv(encoding : Encoding.UTF8, quoting : false);

OUTPUT @result2
TO @output2
USING Outputters.Csv(encoding : Encoding.UTF8, quoting : false);
...

U-SQL does not support dynamic U-SQL directly but there are ways of doing it. Two example ways of doing it are here and here.

Answer 2

As of 2020, this is released behind a Feature Preview flag DataPartitionedOutput:on

SET @@FeaturePreviews = "DataPartitionedOutput:on";


@rows =
EXTRACT 
Firstname string,
Lastname string,
Score string
FROM "/input/a.csv"
USING Extractors.Csv(skipFirstNRows:1);


OUTPUT @rows
TO @"output/{Score}.csv"
USING Outputters.Csv();