SELECT 
  category_id, 
  yml2_market_category 
FROM 
  cscart_categories 
WHERE 
  category_id IN ('478') 
  AND yml2_market_category != ''

Query time 0.00022

JSON explain

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