Executes an OPTIMIZE TABLE statement on all WordPress MySQL database tables, for instance from within a plugin: https://www.saotn.org/optimize-wordpress-mysql-tables-cron/

saotn-wpdb-optimizer.php

/**
 * Executes an OPTIMIZE TABLE statement on all WordPress database 
 * tables.
 */
public static function saotn_wpdb_optimizer() {
  global $wpdb;
  $tables = $wpdb->get_col( "SHOW TABLES" );
  foreach ( $tables as $table ) {
    $wpdb->query( "OPTIMIZE TABLE $table" );
  }
}