C#

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//Read the file in and get just the key field along with
//some tracking information to find the line later.
//Getting the lines one at a time instead of all at once like ReadAllLines
//is an exercise left to the reader.
IEnumerable<string> fileEnumer = System.IO.File.ReadAllLines("input.txt");
var rows = (from line in fileEnumer
            select new LineInfo(float.Parse(line.Split(',')[0]), line.Length))
           .ToList();
//Calculate the starting position of each line in the file
long curPos = 0;
foreach (var lineInfo in rows)
{
    lineInfo.LineStart = curPos;
    curPos += lineInfo.LineLength;
}
//Sort the lines by Id
rows.Sort((left, right) => left.Id.CompareTo(right.Id));
//Copy the lines into a new file
using (var inStream = new System.IO.FileStream("input.txt", 
                                               System.IO.FileMode.Open, 
                                               System.IO.FileAccess.Read, 
                                               System.IO.FileShare.None))
{
    using (var reader = new System.IO.StreamReader(inStream))
    {
        using (var output = new System.IO.StreamWriter("output.txt"))
        {
            foreach (var lineInfo in rows)
            {
                inStream.Seek(lineInfo.LineStart, System.IO.SeekOrigin.Begin);
                output.WriteLine(reader.ReadLine());
            }
        }
    }
}

public class LineInfo
{
    public LineInfo(float id, int lineLength)
    {
        this.Id = id;
        this.LineLength = lineLength;
    }
    public float Id { get; private set; }
    public int LineLength { get; private set; }
    public long LineStart { get; set; }
}

public static IEnumerable<TResult> 
    SelectWithPrevious<TSource, TResult>(this IEnumerable<TSource> source,
                                         Func<TSource, TResult, TResult> selector,
                                         TResult firstPrevious)
{
    foreach (var item in source)
    {
        firstPrevious = selector(item, firstPrevious);
        yield return firstPrevious;
    }
}

IEnumerable<string> fileEnumer = null; /* = get lines one at a time */ ;
var rows = from row in fileEnumer
           .SelectWithPrevious((line, previous) => new {Id = float.Parse(line.Split(',')[0]), 
                                                        LineLength = line.Length, 
                                                        LineStart = previous.LineStart + previous.LineLength}, 
                               new {Id = 0.0F, LineLength = 0, LineStart = 0})
           orderby row.Id
           select row;

Gideon Engelberth wrote:

This is how I would implement Luc's last suggestion

Gideon Engelberth wrote:

System.IO.File.ReadAllLines("input.txt");

Gideon Engelberth wrote:

//Read the file in and get just the key field along with
//some tracking information to find the line later.
//Getting the lines one at a time instead of all at once like ReadAllLines
//is an exercise left to the reader.
IEnumerable<string> fileEnumer = System.IO.File.ReadAllLines("input.txt");

BobInNJ wrote:

Also, I do not have a database system