If you’re noticing high MySQL/MariaDB load, slow queries, or “Sending data” processes when running WordPress, especially on WooCommerce or sites with many posts/products, you might be hitting this problem.

Common Symptoms

When you run SHOW PROCESSLIST;, you might see dozens or hundreds of queries stuck like this:

SELECT SQL_CALC_FOUND_ROWS wp_posts.ID
FROM wp_posts
LEFT JOIN wp_term_relationships ON (wp_posts.ID = wp_term_relationships.object_id)
WHERE ...

• Status: Sending data
• Time: Hundreds or thousands of seconds
• Command: Query
• State: Sending data

This is a clear indicator that WordPress is missing a proper index for efficient joins.

Root Cause

By default, WordPress does not create a standalone index on object_id inside the wp_term_relationships table.

Instead, it only sets a combined primary key:

PRIMARY KEY (object_id, term_taxonomy_id)

This works fine when WordPress is looking up by both object_id and term_taxonomy_id, but it is terrible for queries that only join on object_id, which is very common in themes, WooCommerce, custom post queries, and plugins.

Without a separate index on object_id, MySQL must scan the full table or the combined key every time — leading to massive performance bottlenecks.

How to Fix It

You simply need to create a separate index on object_id in the wp_term_relationships table.

Run this SQL command:

ALTER TABLE wp_term_relationships ADD INDEX idx_object_id (object_id);

If this still doesn’t help, in my case it reduced query time but wasn’t enough, add this to your functions.php

// Globally remove SQL_CALC_FOUND_ROWS from all WPDB queries
add_filter('query', function($query) {
    if (stripos($query, 'SQL_CALC_FOUND_ROWS') !== false) {
        // You can log output if you want
        // error_log('Intercepted query: ' . $query);

        // Remove SQL_CALC_FOUND_ROWS from query
        $query = str_ireplace('SQL_CALC_FOUND_ROWS', '', $query);
    }
    return $query;
});

Replace wp_ with your actual WordPress table prefix if it’s different.

This operation is safe and non-destructive. It will not affect existing data or WordPress functionality. It just improves query speed.

Immediate Benefits

• SELECT ... LEFT JOIN wp_term_relationships ON (object_id) becomes instant instead of dragging for minutes or hours.
• Server CPU and Disk IO usage drops significantly.
• MariaDB/MySQL stops accumulating hundreds of “Sending data” processes.
• WordPress pages, product listings, and category queries load much faster.

Bonus Tip: Avoid SQL_CALC_FOUND_ROWS

Many slow WordPress queries also use SQL_CALC_FOUND_ROWS, which is deprecated and inefficient.

Consider refactoring your queries to:

• Run a SELECT COUNT(*) separately for total counts
• Use LIMIT and OFFSET without needing FOUND_ROWS()

This improves performance even further!

Final Thoughts

This small database tweak — adding a missing index — can save your server from extreme slowdowns.
If you’re seeing Sending data issues in SHOW PROCESSLIST;, always check for missing indexes first.