SELECT 
  products.product_id 
FROM 
  cscart_products AS products 
  LEFT JOIN cscart_warehouses_destination_products_amount AS war_amounts ON war_amounts.product_id = products.product_id 
WHERE 
  products.master_product_id = 759 
  AND (
    (
      products.is_stock_split_by_warehouses = 'Y' 
      AND war_amounts.destination_id = 11 
      AND war_amounts.amount > 0
    ) 
    OR products.is_stock_split_by_warehouses = 'N'
  )

Query time 0.00023

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.00524233,
    "nested_loop": [
      {
        "table": {
          "table_name": "products",
          "access_type": "ref",
          "possible_keys": ["idx_master_product_id"],
          "key": "idx_master_product_id",
          "key_length": "4",
          "used_key_parts": ["master_product_id"],
          "ref": ["const"],
          "loops": 1,
          "rows": 1,
          "cost": 0.00345856,
          "filtered": 100,
          "attached_condition": "products.is_stock_split_by_warehouses = 'Y' or products.is_stock_split_by_warehouses = 'N'"
        }
      },
      {
        "table": {
          "table_name": "war_amounts",
          "access_type": "ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "ref": ["u428615623_ecartifygonje.products.product_id"],
          "loops": 1,
          "rows": 1,
          "cost": 0.00178377,
          "filtered": 100,
          "attached_condition": "trigcond(war_amounts.destination_id = 11 and products.is_stock_split_by_warehouses = 'Y' and war_amounts.amount > 0 or products.is_stock_split_by_warehouses = 'N')"
        }
      }
    ]
  }
}