How does ExecuteReader Work

Question

0.00/5 (No votes)

See more:

Hi

If an application is running on a server "ServerA" and the database is on another server "ServerB", and the code is as below :

Method in application

C#

private static void CreateCommand(string queryString,
    string connectionString)
{
    using (SqlConnection connection = new SqlConnection(
               connectionString))
    {
        connection.Open();

        SqlCommand command = new SqlCommand(queryString, connection);
        SqlDataReader reader = command.ExecuteReader();
        while (reader.Read())
        {
            if (UseFilePartValidate(reader))
            {
               AddPartsToFile(reader);
            }
        }
    }
}

Method called with queryString parameter set to

SQL

SELECT FileName, FilePart, FileID FROM SomeFileTable ORDER BY FilePartOrder

where will this do it's filtering of the data, ServerA or ServerB? If ServerA does that mean it will pull all the data across to ServerA, as in all the file parts of all the files in that table?

I assume this will bring all the data across to ServerA but lately I've seen with a lot of older projects that it works as in the example above, especially with Collections and just thought maybe I was missing something.

Thanks in advance

Posted 24-Jan-12 23:36pm

Gordon Beeming

Updated 24-Jan-12 23:41pm

v3

Add a Solution

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

OriginalGriff · Answer 1 · 2012-01-25T05:02:00

Since your command doesn't do any filtering, none of them!

If your command was:

SQL

SELECT FileName, FilePart, FileID FROM SomeFileTable WHERE FileID < 20 ORDER BY FilePartOrder

Then ServerB will do the filtering, since it receives the whole command. That is one of the reasons why you shouldn't say

SQL

SELECT * FROM myTable

instead of specifying the fields you want as in your example. It wastes bandwidth on the transfer as well as memory on the server - this can be significant when your table contains images for example.

This assumes you are using SQL Server, or MySQL: if you are using Access for some strange reason, then all the data has to be pulled to ServerA as the command is executed there so any filtering will be done there.