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SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = 'cf5307e427e1a2d7ba8425b7ed1a2c86b09294a29e8e1af013483df2e9a9d5c1' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00023

JSON explain

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