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SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = '5d8c4f8d7c3385d1f3d9ef847b11ed5eccb3978902fc0586408e0fb10af441a4' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00018

JSON explain

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