WordPress: Reduce Database Queries

functions.php

Reduce database queries (http://www.catswhocode.com/blog/speeding-up-your-wordpress-blog-7-effective-solutions)
It is important to reduce unecessary queries to your database as each query take a few milliseconds to execute. First, you might want to know how many queries your blog execute in order to display a page. To do so, paste the code below in your functions.php file. Once done, just have a look to your site footer to know how many queries has been executed and how many time it took to completely load the page.

add_action( 'wp_footer', 'tcb_note_server_side_page_speed' );
function tcb_note_server_side_page_speed() {
  date_default_timezone_set( get_option( 'timezone_string' ) );
  $content  = '[ ' . date( 'Y-m-d H:i:s T' ) . ' ] ';
  $content .= 'Page created in ';
  $content .= timer_stop( $display = 0, $precision = 2 );
  $content .= ' seconds from ';
  $content .= get_num_queries();
  $content .= ' queries';
  if( ! current_user_can( 'administrator' ) ) $content = "<!-- $content -->";
  echo $content;
}
Then, you have to remove useless queries from your blog. Start by making sure that you are not using too many plugins, as most plugins are making database queries. Then you can remove theme-related queries that are not useful to your blog.