Keep Database Queries Out of Templates — Use pre_get_posts

<!-- single.php -->
<?php
// ❌ query_posts() overwrites the main query — the original query was wasted
query_posts( 'post_type=product&posts_per_page=10' );
while ( have_posts() ) : the_post();
    the_title();
endwhile;
// Pagination is broken, global state is corrupted
?>
 
<!-- sidebar.php -->
<?php
// ❌ Direct SQL in template — bypasses caching and object permissions
global $wpdb;
$popular = $wpdb->get_results(
    "SELECT ID, post_title FROM wp_posts
     WHERE post_type = 'post' AND post_status = 'publish'
     ORDER BY RAND() LIMIT 5"
);
// ORDER BY RAND() = full table scan on every page load
?>
 
<!-- archive.php -->
<?php
// ❌ Custom query overriding the main loop instead of modifying it
$custom = new WP_Query([
    'post_type'      => 'post',
    'posts_per_page' => 12,
    'category_name'  => 'featured',
]);
// The original archive query already ran and was thrown away
?>

// functions.php — modify the main query BEFORE it runs
function my_theme_customize_main_query( WP_Query $query ) {
    if ( is_admin() || ! $query->is_main_query() ) {
        return; // Only modify the main frontend query
    }
 
    // Customize archive pages
    if ( $query->is_archive() ) {
        $query->set( 'posts_per_page', 12 );
    }
 
    // Customize category pages
    if ( $query->is_category( 'featured' ) ) {
        $query->set( 'orderby', 'date' );
        $query->set( 'order', 'DESC' );
    }
 
    // Customize search
    if ( $query->is_search() ) {
        $query->set( 'post_type', [ 'post', 'page', 'product' ] );
    }
}
add_action( 'pre_get_posts', 'my_theme_customize_main_query' );

// ✅ When a secondary query IS needed (sidebar widgets, related posts),
// use WP_Query (NEVER query_posts) and reset properly
<?php
$related = new WP_Query([
    'post_type'      => 'post',
    'posts_per_page' => 5,
    'post__not_in'   => [ get_the_ID() ],
    'category__in'   => wp_get_post_categories( get_the_ID() ),
    'no_found_rows'  => true, // Skip pagination count — saves one query
]);
 
if ( $related->have_posts() ) :
    while ( $related->have_posts() ) : $related->the_post();
        get_template_part( 'template-parts/card' );
    endwhile;
    wp_reset_postdata(); // Always reset after secondary WP_Query loops
endif;
?>

// ✅ Instead of ORDER BY RAND(), use a cached random selection
function get_random_posts_cached( $count = 5 ) {
    $cache_key = 'random_posts_' . $count;
    $cached = get_transient( $cache_key );
 
    if ( false !== $cached ) {
        return $cached;
    }
 
    $posts = get_posts([
        'post_type'   => 'post',
        'numberposts' => $count,
        'orderby'     => 'rand',
        'no_found_rows' => true,
    ]);
 
    set_transient( $cache_key, $posts, 15 * MINUTE_IN_SECONDS );
    return $posts;
}

# Find query_posts (almost always wrong)
grep -rn "query_posts\s*(" wp-content/themes/ wp-content/plugins/ --include="*.php"
# Find direct $wpdb in template files
grep -rn "\$wpdb->" wp-content/themes/ --include="*.php"
# Find ORDER BY RAND
grep -rn "ORDER BY RAND\|orderby.*rand" wp-content/ --include="*.php"