how to increase performance of mysql query?

Question

I'm using this query to check the username is already exist or not in database using ajax so till now i hv only few records in DB bt in future username list gonna be huge so i want to improve the performance of query in such a way it will take less time to check username existence.

how can i improve following query ? is indexing on db is feasible solution for this ?

check.php

<?php
if(isset($_POST['user_name']))
{
$user_name=$_POST['user_name'];
include("include/conn.php");
$sql_check = mysql_query("select userid from `vector`.`signup` where userid='".$user_name."'")
 or die(mysql_error());

if (mysql_num_rows($sql_check)>0)
{   
    echo "no";
} 
else
{
 echo "yes";
}
}
?> 

my jquery code 

<script language="javascript">

$(document).ready(function()
{
    $("#uname").blur(function()
    {
                $("#msgbox").removeClass().addClass('messagebox').text('Checking...').fadeIn("slow");

        $.post("check.php",{ user_name:$(this).val() } ,function(data)
        {
          if(data=='no') //if username not avaiable
          {
            $("#msgbox").fadeTo(200,0.1,function() 
            { 

              $(this).html('This User name Already exists').addClass('messageboxerror').fadeTo(900,1);
            });     
          }
          else
          {
            $("#msgbox").fadeTo(200,0.1,function()  
            { 

              $(this).html('Username available to register').addClass('messageboxok').fadeTo(900,1);    
            });
          }

        });

    });
});
</script>

Answer 1

Creating an index on the userid column of the vector.signup table will probably solve your performance issues.

As it stands you have a massive security flaw in your script. You should never inject POST data directly into a query, because you open yourself up to a SQL injection attack. You should escape your data first:

$user_name = mysql_real_escape_string($_POST['user_name']);

You can read more here:

http://php.net/manual/en/function.mysql-real-escape-string.php