EF Core 和 MySql 查询速度太慢

Question

我在 mysql 中有 2 个 table 命名的市场和交易所。
交易所中大约有 150 万条记录 table.
我想从交易所 table 获取每个市场更新日期最长的记录。作为查询的结果，我想要 return 一个列表，每个市场只有一个最近的更新。
我使用 linq 创建了如下查询。但是他反应很慢。
大约 7-8 分钟。我该如何解决这个问题？

感谢您的回答。

var query = (from exchange in _context.Exchanges
             join market in _context.Markets on exchange.MarketId equals market.Id
             where market.TypeId == 1
             group exchange by exchange.MarketId into grp

             let maxdate = grp.Max(x => x.LastUpdatedDateTime)

             from exchanges in grp
             where exchanges.LastUpdatedDateTime == maxdate
             select new DtoGetAllMarketsWithLastExchanges
             {
                 Id = exchanges.MarketId,
                 Code = exchanges.Markets.Code,
                 Name = exchanges.Markets.Name,
                 LastBuyPrice = exchanges.LastBuyPrice,
                 LastSellPrice = exchanges.LastSellPrice,
                 SeoUrl = exchanges.Markets.SeoUrl,
                 Icon = exchanges.Markets.Icon,
                 LastUpdateDate = exchanges.LastUpdatedDateTime,
                 Rate = exchanges.Rate
             }).ToList();

市场实体 Class;

[Table("Markets")]
public partial class Markets : BaseEntity
{
    public int TypeId { get; set; }

    public string Code { get; set; }

    public string Name { get; set; }

    public byte Unit { get; set; }

    public int OrderNumber { get; set; }

    public string Icon { get; set; }

    public string SeoUrl { get; set; }

    public virtual List<Exchanges> Exchanges { get; set; }
}

交换实体 Class;

[Table("Exchanges")]
public partial class Exchanges : BaseEntity
{
    public int MarketId { get; set; }

    public double LastBuyPrice { get; set; }

    public double LastSellPrice { get; set; }

    public double Rate { get; set; }

    public DateTime CreatedDateTime { get; set; }

    public DateTime LastUpdatedDateTime { get; set; }

    [ForeignKey("MarketId")]
    public virtual Markets Markets { get; set; } 

}

Answer 1

查询速度很慢，因为它使用了当前不受支持的结构，这会导致 client evaluation. Client evaluation is quite inefficient with big data, which is one of the reasons it will be removed in EF Core 3.0。

所以诀窍是找到支持的 LINQ 构造（转换为 SQL 并在服务器端执行）。由于每个 EF Core 版本都支持不同的构造，我不能确切地说出 2.0 支持哪一个，但在最新的 2.2 中，一般来说，请尝试遵循以下简单规则：

尽可能使用导航属性而不是手动连接
尽可能避免GroupBy。

并始终检查客户评估警告。在 3.0+ 中，它们无论如何都是运行时异常，所以你必须找到一个可翻译的结构。

以下是在 2.2.6 中工作的一些功能上 等效的 LINQ 查询：

(1) 符合 Max 标准

from market in _context.Markets
where market.TypeId == 1
from exchange in market.Exchanges
where exchange.LastUpdatedDateTime == market.Exchanges.Max(e => (DateTime?)e.LastUpdatedDateTime)
select new DtoGetAllMarketsWithLastExchanges
{
    Id = market.Id,
    Code = market.Code,
    Name = market.Name,
    LastBuyPrice = exchange.LastBuyPrice,
    LastSellPrice = exchange.LastSellPrice,
    SeoUrl = market.SeoUrl,
    Icon = market.Icon,
    LastUpdateDate = exchange.LastUpdatedDateTime,
    Rate = exchange.Rate
}

(2) 与 (1) 相同，但具有 !Any (SQL NOT EXISTS) 条件：

where !market.Exchanges.Any(e => e.LastUpdatedDateTime < exchange.LastUpdatedDateTime)

(3) 与 (1) 相同，但具有 join 到 Max 子查询：

from market in _context.Markets
where market.TypeId == 1
from exchange in market.Exchanges
join lastExchange in _context.Exchanges
    .GroupBy(e => e.MarketId)
    .Select(g => new { MarketId = g.Key, Date = g.Max(e => e.LastUpdatedDateTime) })
on new { exchange.MarketId, Date = exchange.LastUpdatedDateTime }
equals new { lastExchange.MarketId, lastExchange.Date }
select new DtoGetAllMarketsWithLastExchanges
{
    Id = market.Id,
    Code = market.Code,
    Name = market.Name,
    LastBuyPrice = exchange.LastBuyPrice,
    LastSellPrice = exchange.LastSellPrice,
    SeoUrl = market.SeoUrl,
    Icon = market.Icon,
    LastUpdateDate = exchange.LastUpdatedDateTime,
    Rate = exchange.Rate
}

EF Core 和 MySql 查询速度太慢

EF Core and MySql query is too slow

c#

mysql

linq

entity-framework-core

ef-core-2.0