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SELECT 
  1 
FROM 
  cscart_lock_keys 
WHERE 
  key_id = '6e5ad91cbc22b341e5c8685b0b99f0f243e4c1318a5e7f4c3a3a79a98459d159' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00023

JSON explain

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