How to read millions of rows from the text file an

Question

I have gone through the Insert 2 million rows into SQL Server quickly link and found that I can do this by using Bulk insert. So I am trying to create the datatable (code as below), but as this is a huge file (more than 300K row) I am getting an OutOfMemoryEexception in my code:

string line;
DataTable data = new DataTable();
string[] columns = null;    
bool isInserted = false;           

using (TextReader tr = new StreamReader(_fileName, Encoding.Default))
{
   if (columns == null)
   {
      line = tr.ReadLine();
      columns = line.Split(',');
   }

   for (int iColCount = 0; iColCount < columns.Count(); iColCount++)
   {
      data.Columns.Add("Column" + iColCount, typeof(string));
   }                       

   string[] columnVal;

   while ((line = tr.ReadLine()) != null)
   {
        columnVal = line.Split(','); // OutOfMemoryException throwing in this line
        data.Rows.Add(columnVal);
    }
}

after long work I modified my code to as below but then also I am getting OutOfMemoryException at the time of adding rows into datatable

 DataTable data = new DataTable();
 string[] columns = null;
 var line = string.Empty;
 using (TextReader tr = new StreamReader(_fileName, Encoding.Default))
 {
     if (columns == null)
     {
         line = tr.ReadLine();
         columns = line.Split(',');
     }

     for (int iColCount = 0; iColCount < columns.Count(); iColCount++)
     {
        data.Columns.Add("Column" + iColCount, typeof(string));
     }
  }

  // Split the rows in 20000 rows in different list

  var _fileList = File.ReadLines(_fileName, Encoding.Default).ToList();
  var splitChunks = new List<List<string>>();
  splitChunks = SplitFile(_fileList, 20000);

 Parallel.ForEach(splitChunks, lstChunks =>
 {
   foreach (var rows in lstChunks)
   {
     string[] lineFields = rows.Split(',');
     DataRow row = datatbl.NewRow();
     for (int iCount = 0; iCount < lineFields.Count(); iCount++)
     {
        row[iCount] = lineFields[iCount] == string.Empty ? "" : lineFields[iCount].ToString();
     }
     datatbl.Rows.Add(row);
   }
 }); 

I can do the bulk insert for next level as the below code:

SqlConnection SqlConnectionObj = GetSQLConnection();
SqlBulkCopy bulkCopy = new SqlBulkCopy(SqlConnectionObj, SqlBulkCopyOptions.TableLock | SqlBulkCopyOptions.FireTriggers | SqlBulkCopyOptions.UseInternalTransaction, null);
bulkCopy.DestinationTableName = "TempTable";
bulkCopy.WriteToServer(data);

File contains below kind of data

4714,1370,AUSRICHTEN MASCHINELL

4870,1370,PLATTE STECKEN

0153,1900,CAULK GUN

0154,1900,NEW TERMINATOR

0360,1470,MU 186 MACCH. X LAV. S/A ASTE PS174

9113-H22,1970,MC DRILL BITS

Code need to convert this into 6 rows and 3 columns.

Is there any faster way to achieve the above functionality to read the file and create the datatable for bulk insert? So that I should not get memory out of index exception.

Thanks in advance.

Answer 1

Solution with SqlBulkCopy.WriteToServer and IDataReader. I'm using CSV, but I hope it will easy to modify for other types. SqlBulkCopy uses just 3 things from IDateReader and we have to implement them:

• public int FieldCount {get; }
• public bool Read()
• public object GetValue(int i)

All other properties and methods can be unimplemented. Interesting paper about SqlBulkCopy. Full code: https://dotnetfiddle.net/giG3Ai. Here is with cutted version:

namespace SqlBulkCopy
{
    using System;
    using System.Collections.Generic;
    using System.IO;
    using System.Diagnostics;
    using System.Data;
    using System.Data.SqlClient;

    public class CsvReader : IDataReader
    {
        private readonly char CSV_DELIMITER = ',';

        private readonly StreamReader _sr;
        private readonly Dictionary<string, Func<string, object>> _csv2SqlType;
        private readonly string[] _headers;

        private string _line;
        private string[] _values;

        public int FieldCount { get { return _headers.Length; } }

        public CsvReader(string filePath, Dictionary<string, Func<string, object>> csvColumn2SqlTypeDict)
        {
            if (string.IsNullOrEmpty(filePath))
                throw new ArgumentException("is null or empty", "filePath");
            if (!System.IO.File.Exists(filePath))
                throw new IOException(string.Format("{0} doesn't exist or access denied", filePath));
            if (csvColumn2SqlTypeDict == null)
                throw new ArgumentNullException("csvColumn2SqlTypeDict");

            _sr = new StreamReader(filePath);
            _csv2SqlType = csvColumn2SqlTypeDict;
            _headers = ReadHeaders();
            ValidateHeaders();
        }
        public object GetValue(int i)
        {
            // Get column value
            var colValue = _values[i];
            // Get column name
            var colName = _headers[i];
            // Try to convert to SQL type
            try { return _csv2SqlType[colName](colValue); }
            catch { return null; }
        }
        public bool Read()
        {
            if (_sr.EndOfStream) return false;

            _line = _sr.ReadLine();
            _values = _line.Split(CSV_DELIMITER);
            // If row is invalid, go to next row
            if (_values.Length != _headers.Length)
                return Read();
            return true;
        }
        public void Dispose()
        {
            _sr.Dispose();
        }
        private void ValidateHeaders()
        {
            if (_headers.Length != _csv2SqlType.Keys.Count)
                throw new InvalidOperationException(string.Format("Read {0} columns, but csv2SqlTypeDict contains {1} columns", _headers.Length, _csv2SqlType.Keys));
            foreach (var column in _headers)
            {
                if (!_csv2SqlType.ContainsKey(column))
                    throw new InvalidOperationException(string.Format("There is no convertor for column '{0}'", column));
            }
        }
        private string[] ReadHeaders()
        {
            var headerLine = _sr.ReadLine();
            if (string.IsNullOrEmpty(headerLine))
                throw new InvalidDataException("There is no header in CSV!");
            var headers = headerLine.Split(CSV_DELIMITER);
            if (headers.Length == 0)
                throw new InvalidDataException("There is no header in CSV after Split!");
            return headers;
        }
    }
    public class Program
    {        
        public static void Main(string[] args)
        {
            // Converter from CSV columns to SQL columns
            var csvColumn2SqlTypeDict = new Dictionary<string, Func<string, object>>
            {
                { "int", (s) => Convert.ToInt32(s) },
                { "str", (s) => s },
                { "double", (s) => Convert.ToDouble(s) },
                { "date", (s) => Convert.ToDateTime(s) },
            };
            Stopwatch sw = Stopwatch.StartNew();
            try
            {
                // example.csv
                /***
                   int,str,double,date
                   1,abcd,2.5,15.04.2002
                   2,dab,2.7,15.04.2007
                   3,daqqb,4.7,14.04.2007
                 ***/
                using (var csvReader = new CsvReader("example.csv", csvColumn2SqlTypeDict))
                {
                    // TODO!!! Modify to your Connection string
                    var cs = @"Server=localhost\SQLEXPRESS;initial catalog=TestDb;Integrated Security=true";
                    using (var loader = new SqlBulkCopy(cs, SqlBulkCopyOptions.Default))
                    {
                        // TODO Modify to your Destination table
                        loader.DestinationTableName = "Test";
                        // Write from csvReader to database
                        loader.WriteToServer(csvReader);
                    }
                }
            }
            catch(Exception ex)
            {
                Console.WriteLine("Got an exception: {0}", ex);
                Console.WriteLine("Press 'Enter' to quit");
                Console.ReadLine();
                return;
            }
            finally { sw.Stop(); }
            Console.WriteLine("Data has been written in {0}", sw.Elapsed);
            Console.WriteLine("Press 'Enter' to quit");
            Console.ReadLine();
        }
        private static void ShowCsv(IDataReader dr)
        {
            int i = 0;
            while (dr.Read())
            {
                Console.WriteLine("Row# {0}", i);
                for (int j = 0; j < dr.FieldCount; j++)
                {
                    Console.WriteLine("{0} => {1}", j, dr.GetValue(j));
                }
                i++;
            }
        }
    }
}

Answer 2

I found forgetting a DataTable and using plain old SQLClient on a row-by-row basis was quicker. Simpler too. This also beat a streaming SQL function that was supposedly the fastest way to get data in a SQL Server DB.

Try it and measure the speed, see if its fast enough for you. If it isn't you could always try to re-format the file (if necessary) and let SQL Server do the work for you using its Bulk Insert.

Answer 3

The reason you are getting OutOfMemoryException is because you are creating an in memory data table and are trying to insert 300K rows into it

This is a lot of data to put in memory.

Instead of this you should do is every certain amount of rows you read from the text file - you need to insert it into the database.

How you do it is up to you, you can use SQL, or bulk copy - but keep in mind, you can't read the entire text file and keep it in memory, so do it in chunks.