EF Core 和 MySql 查询太慢

Question

I have 2 tables named markets and exchanges in mysql.
There are approximately 1.5 million records in the exchanges table.
I would like to get records from exchanges table with the highest update date of each market. As a result of the query, I want to return a list with only one recent update for each market.
I created a query like the following with linq. But he responds very slowly.
Approximately 7-8 minutes. How do I fix this problem?

Thanks for your answers.

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();

Markets Entity 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; }
}

Exchanges Entity 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

The query is slow because it uses currently unsupported constructs which cause 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 .

So the trick is to find LINQ construct which is supported (translated to SQL and executed server side). Since every EF Core version supports different constructs, I can't say exactly which one is supported in 2.0, but in latest 2.2 and in general, try following the next simple rules:

• Use navigation properties over manual joins where possible
• Avoid GroupBy where possible.

And always check for client evaluation warnings. In 3.0+ they will be runtime exceptions anyway, so you have to find a translatable construct.

Here are some of the functionally equivalent LINQ queries which work in 2.2.6:

(1) With Max criteria

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) Same as (1), but with !Any (SQL NOT EXISTS ) criteria:

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

(3) Same as (1), but with join to Max subquery:

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
}