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SELECT 
  1 
FROM 
  cscart_ec_print_temp_designs 
WHERE 
  session_id = '9DVV-NMUM-WOJJ-ZGV2-V5RR-S0VF-C4MS-H5T1' 
  AND product_id = 16 
  AND expires_at > 1770180730 
  AND confirmed = 'Y' 
LIMIT 
  1

Query time 0.00035

JSON explain

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