Cache Expensive Operations with Transients

Impact: HIGH (reduces page load time by 200-2000ms for pages making external API calls or complex queries)

WordPress transients are key-value pairs with an expiration time, stored in wp_options (or in a persistent object cache like Redis if configured). Any operation that is slow and returns the same result for a period of time should be cached: external API calls, complex database aggregations, third-party SDK queries, and computationally expensive transformations.

Incorrect (uncached expensive operations on every request):

// ❌ HTTP request on every page load — 200-2000ms latency each time
function get_exchange_rates() {
    $response = wp_remote_get( 'https://api.exchangerate.host/latest' );
    if ( is_wp_error( $response ) ) {
        return [];
    }
    return json_decode( wp_remote_retrieve_body( $response ), true );
}
 
// ❌ Complex aggregation query on every request
function get_sales_stats() {
    global $wpdb;
    return $wpdb->get_results(
        "SELECT DATE(post_date) as date, COUNT(*) as orders, SUM(meta_value) as revenue
         FROM {$wpdb->posts} p
         JOIN {$wpdb->postmeta} pm ON p.ID = pm.post_id AND pm.meta_key = '_order_total'
         WHERE p.post_type = 'shop_order' AND p.post_status = 'wc-completed'
         GROUP BY DATE(post_date)
         ORDER BY date DESC
         LIMIT 30"
    );
    // Runs a JOIN + GROUP BY on every page load
}

Correct (transient caching with proper patterns):

// ✅ Cache external API calls
function get_exchange_rates() {
    $cache_key = 'my_plugin_exchange_rates';
    $cached = get_transient( $cache_key );
 
    if ( false !== $cached ) {
        return $cached; // Cache hit
    }
 
    $response = wp_remote_get( 'https://api.exchangerate.host/latest', [
        'timeout' => 10,
    ]);
 
    if ( is_wp_error( $response ) ) {
        // On failure, try to use expired cache (stale-while-revalidate pattern)
        $stale = get_option( '_transient_stale_' . $cache_key );
        return $stale ?: [];
    }
 
    $data = json_decode( wp_remote_retrieve_body( $response ), true );
 
    // Cache for 1 hour; store a stale copy as backup
    set_transient( $cache_key, $data, HOUR_IN_SECONDS );
    update_option( '_transient_stale_' . $cache_key, $data, 'no' ); // autoload=no
 
    return $data;
}

// ✅ Cache complex queries with cache invalidation on data change
function get_sales_stats() {
    $cache_key = 'my_plugin_sales_stats';
    $cached = get_transient( $cache_key );
 
    if ( false !== $cached ) {
        return $cached;
    }
 
    global $wpdb;
    $results = $wpdb->get_results(
        $wpdb->prepare(
            "SELECT DATE(post_date) as date, COUNT(*) as orders, SUM(meta_value) as revenue
             FROM {$wpdb->posts} p
             JOIN {$wpdb->postmeta} pm ON p.ID = pm.post_id AND pm.meta_key = %s
             WHERE p.post_type = %s AND p.post_status = %s
             GROUP BY DATE(post_date)
             ORDER BY date DESC
             LIMIT 30",
            '_order_total',
            'shop_order',
            'wc-completed'
        )
    );
 
    set_transient( $cache_key, $results, 15 * MINUTE_IN_SECONDS );
    return $results;
}
 
// Invalidate when new orders are placed
add_action( 'woocommerce_order_status_completed', function() {
    delete_transient( 'my_plugin_sales_stats' );
});

WordPress time constants:

Constant	Seconds
`MINUTE_IN_SECONDS`	60
`HOUR_IN_SECONDS`	3,600
`DAY_IN_SECONDS`	86,400
`WEEK_IN_SECONDS`	604,800

When NOT to use transients:

Data that changes on every request (user-specific session data)
Small/fast operations (a single indexed query is faster than transient overhead)
Sensitive data (transients are stored in wp_options, accessible on shared object caches)

Detection hints:

# Find API calls without transient caching
grep -rn "wp_remote_get\|wp_remote_post" wp-content/plugins/ --include="*.php" -l | xargs grep -L "get_transient\|set_transient"
# Find complex queries without caching
grep -rn "GROUP BY\|JOIN.*JOIN\|UNION" wp-content/plugins/ --include="*.php" -l | xargs grep -L "transient\|wp_cache"

Reference: Transients API · WordPress Performance Best Practices

Cache Expensive Operations with Transients

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Cache Expensive Operations with Transients