Adds 1M actions to the Action Scheduler queue, with 20K of them already due, and another 100 coming due each minute.

fill-the-db.php

<?php

function action_scheduler_load_testing() {
	/** @var wpdb $wpdb */
	global $wpdb;

	$sql = "INSERT INTO {$wpdb->posts} (post_author, post_date, post_date_gmt, post_content, post_title, post_status, post_name, post_modified, post_modified_gmt, post_type) VALUES ";
	for ( $i = -200 ; $i < 9800 ; $i++ ) {
		$date = new DateTime($i.' minutes');
		$action = md5($i);
		$fields = array(
			0,
			$date->format('Y-m-d H:i:s'),
			$date->format('Y-m-d H:i:s'),
			json_encode(array($i)),
			$action,
			'pending',
			$action,
			$date->format('Y-m-d H:i:s'),
			$date->format('Y-m-d H:i:s'),
			'scheduled-action',
		);
		$fields_sql = vsprintf( "( %d, '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s', '%s' )", $fields );
		$values = array_fill( 0, 100, $fields_sql );
		$this_sql = $sql . implode(', ', $values);
		$wpdb->query($this_sql);
	}
}

include_once dirname(dirname(dirname(dirname(dirname(__FILE__))))).'/wp/wp-load.php';

action_scheduler_load_testing();