c# – 单个查询慢于3个查询

我首先使用代码生成此产品类：

public class Part
{
    public int PartID { get; set; }

    [Required(ErrorMessage = "xxx")]        
    public string Title { get; set; }

    [MaxLength(50)]
    [Index(IsClustered = false,IsUnique = false,Order =1)]
    public string Part_Number { get; set; }

    [MaxLength(50)]
    [Index(IsClustered = false,Order = 2)]
    public string Manufacturer_Number { get; set; }
}

我在数据库中有大约250万个这样的实体.

第一种方法

var query = db.Parts.Where(s => s.Manufacturer_Number == sstring).ToList();                
query.AddRange(db.Parts.Where(s => s.Part_Number == sstring).ToList());
query.AddRange(db.Parts.Where(s => s.Title == sstring).ToList());

第二种方法

var query = db.Parts.Where(s => s.Manufacturer_Number == sstring
|| s.Part_Number == sstring || s.Title == sstring).ToList();

第一种方法比第二种方法快100倍.有谁能解释一下？