SELECT 
  ticket_id 
FROM 
  cscart_messenger_tickets 
WHERE 
  status = 'N' 
  AND unread_participant_id > 0 
  AND unread_started_at > 0 
  AND unread_initial_email_sent_at > 0 
  AND unread_reminder_sent_at = 0 
  AND unread_started_at <= 1788327103 
ORDER BY 
  unread_started_at ASC 
LIMIT 
  20

Query time 0.00012

JSON explain

{
  "query_block": {
    "select_id": 1,
    "read_sorted_file": {
      "filesort": {
        "sort_key": "cscart_messenger_tickets.unread_started_at",
        "table": {
          "table_name": "cscart_messenger_tickets",
          "access_type": "ALL",
          "rows": 17,
          "filtered": 100,
          "attached_condition": "cscart_messenger_tickets.unread_reminder_sent_at = 0 and cscart_messenger_tickets.`status` = 'N' and cscart_messenger_tickets.unread_participant_id > 0 and cscart_messenger_tickets.unread_started_at > 0 and cscart_messenger_tickets.unread_initial_email_sent_at > 0 and cscart_messenger_tickets.unread_started_at <= 1788327103"
        }
      }
    }
  }
}