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SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = 'b39c265b5defeb0e289b6e5c9778cc979b29f80d1cc39925cce6df4a5e31ed91' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00026

JSON explain

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