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SELECT 
  s.*, 
  d.slide_name 
FROM 
  cscart_ec_print_slides s 
  LEFT JOIN cscart_ec_print_slide_descriptions d ON s.slide_id = d.slide_id 
  AND d.lang_code = 'en' 
WHERE 
  s.product_id = 49 
ORDER BY 
  s.position, 
  s.slide_id

Query time 0.00023

JSON explain

{
  "query_block": {
    "select_id": 1,
    "message": "Impossible WHERE noticed after reading const tables"
  }
}