title: Does the number of columns affect query performance?
body: <p>CASE 1: I have a table with 30 columns and  I query using 4 columns in the where clause.</p>
  
  <p>CASE 2: I have a table with 6 columns and I query using 4 columns in the where clause.</p>
  
  <p>What is the difference in performance in both cases?</p>
  
  <p>For example  i have table</p>
  
  <pre><code>table A
  {
    b varchar(10),
    c varchar(10),
    d varchar(10),
    e varchar(10),
    f varchar(10),
    g varchar(10),
    h varchar(10)
  
  }
  
  SELECT b,c,d
  FROM A
  WHERE f='foo'
  
  create table B
  {
    b varchar(10),
    c varchar(10),
    d varchar(10),
    e varchar(10),
    f varchar(10)
  
  }
  
  SELECT b,c,d
  FROM B
  WHERE f='foo'
  </code></pre>
  
  <p>Both A And B table have same structure means only difference in number of column and column used in where condition is also same and column in select is also same. difference is that table B only have some unused column these are not being used in select and where condition
  in that case is there any difference in performance of both queries ?</p>