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SELECT 
  1 
FROM 
  cscart_multi_declock_keys 
WHERE 
  key_id = 'b51275887284517adc0d1f46af0125a1b01ffdcfd2e52b05b760149a0b4d4381' 
  AND token = '6Hfso2QuxX3bUYzedurfrZyzIqiuRdyEHmaXpXmRKwo=' 
  AND expiry_at > UNIX_TIMESTAMP(NOW())

Query time 0.00023

JSON explain

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