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SELECT 
  * 
FROM 
  cscart_subscribers 
  INNER JOIN cscart_user_mailing_lists ON cscart_subscribers.subscriber_id = cscart_user_mailing_lists.subscriber_id 
WHERE 
  cscart_subscribers.email = ''

Query time 0.00045

JSON explain

{
  "query_block": {
    "select_id": 1,
    "message": "no matching row in const table"
  }
}