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SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = '856a76ed5a775da8f0fd35c850d80785c86b1b8e34f8cf34ead64db83b20c863' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00019

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE								Impossible WHERE noticed after reading const tables