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SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = 'e31e296646ed6e2f834e7617f39987b5232e409f955ec5a1f644788786d43265' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00010

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