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SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = '521c3b5c65d1fe6085e572eb8341c77f94ba8afd8ab0882ce06a009ce01b4b05' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00017

JSON explain

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