问题描述
来自 postgresql：
select client_id,sum(net_amount) from purchases
left join accounts on accounts.id= contracts.account_id
where purchase_date <= '2020-05-08'
and purchase_date > '2019-05-08'
group by client_id
这段代码只用了大约 10 秒的时间来执行，但是下面的 C# 代码失败了：
来自 C#/.Net：
var clientPurchases = _context.Purchases
.Where(m => accountIds.Contains(m.AccountId) &&
m.Date >= resultingStartDate &&
m.Date <= resultingEndDate)
.ToList();
var sum = clientPurchases.Sum(m=>m.NetAmount);
这是抛出的消息：
Unhandled Exception: System.Data.Entity.Core.EntityCommandExecutionException: An error occurred while executing the command deFinition. See the inner exception for details. ---> Npgsql.NpgsqlException: Exception while reading from stream ---> System.IO.IOExcept
ion: Unable to read data from the transport connection: A connection attempt Failed because the connected party did not properly respond after a period of time,or established connection Failed because connected host has Failed to respond. ---> System.Net.socke
ts.socketException: A connection attempt Failed because the connected party did not properly respond after a period of time,or established connection Failed because connected host has Failed to respond
at System.Net.sockets.socket.Receive(Byte[] buffer,Int32 offset,Int32 size,SocketFlags socketFlags)
at System.Net.sockets.NetworkStream.Read(Byte[] buffer,Int32 size)
--- End of inner exception stack trace ---
at System.Net.sockets.NetworkStream.Read(Byte[] buffer,Int32 size)
at Npgsql.NpgsqlReadBuffer.<>c__displayClass31_0.<<Ensure>g__EnsureLong|0>d.MoveNext()
--- End of inner exception stack trace ---
at Npgsql.NpgsqlReadBuffer.<>c__displayClass31_0.<<Ensure>g__EnsureLong|0>d.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Npgsql.NpgsqlConnector.<>c__displayClass161_0.<<ReadMessage>g__ReadMessageLong|0>d.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Threading.Tasks.ValueTask`1.get_Result()
at Npgsql.NpgsqlDataReader.<NextResult>d__46.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Npgsql.NpgsqlDataReader.NextResult()
at Npgsql.NpgsqlCommand.<ExecuteDbDataReader>d__100.MoveNext()
--- End of stack trace from prevIoUs location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Npgsql.NpgsqlCommand.ExecuteDbDataReader(CommandBehavior behavior)
at System.Data.Common.DbCommand.ExecuteReader(CommandBehavior behavior)
at System.Data.Entity.Infrastructure.Interception.DbCommanddispatcher.<>c.<Reader>b__6_0(DbCommand t,DbCommandInterceptionContext`1 c)
at System.Data.Entity.Infrastructure.Interception.Internaldispatcher`1.dispatch[TTarget,TInterceptionContext,TResult](TTarget target,Func`3 operation,TInterceptionContext interceptionContext,Action`3 executing,Action`3 executed)
at System.Data.Entity.Infrastructure.Interception.DbCommanddispatcher.Reader(DbCommand command,DbCommandInterceptionContext interceptionContext)
at System.Data.Entity.Internal.InterceptableDbCommand.ExecuteDbDataReader(CommandBehavior behavior)
at System.Data.Common.DbCommand.ExecuteReader(CommandBehavior behavior)
at System.Data.Entity.Core.EntityClient.Internal.EntityCommandDeFinition.ExecuteStoreCommands(EntityCommand entityCommand,CommandBehavior behavior)
--- End of inner exception stack trace ---
at System.Data.Entity.Core.EntityClient.Internal.EntityCommandDeFinition.ExecuteStoreCommands(EntityCommand entityCommand,CommandBehavior behavior)
at System.Data.Entity.Core.Objects.Internal.ObjectQueryExecutionPlan.Execute[TResultType](ObjectContext context,ObjectParameterCollection parameterValues)
at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__displayClass41_0.<GetResults>b__1()
at System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction[T](Func`1 func,IDbExecutionStrategy executionStrategy,Boolean startLocalTransaction,Boolean releaseConnectionOnSuccess)
at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__displayClass41_0.<GetResults>b__0()
at System.Data.Entity.Infrastructure.DefaultExecutionStrategy.Execute[TResult](Func`1 operation)
at System.Data.Entity.Core.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
at System.Data.Entity.Core.Objects.ObjectQuery`1.<System.Collections.Generic.IEnumerable<T>.GetEnumerator>b__31_0()
at System.Data.Entity.Internal.LazyEnumerator`1.MoveNext()
at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source)
at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.<>c__14`1.<GetElementFunction>b__14_3(IEnumerable`1 sequence)
at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.ExecuteSingle[TResult](IEnumerable`1 query,Expression queryRoot)
at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute[TResult](Expression expression)
at System.Data.Entity.Internal.Linq.DbQueryProvider.Execute[TResult](Expression expression)
at System.Linq.Queryable.Sum[TSource](IQueryable`1 source,Expression`1 selector)
我正在尝试对大约 10000 个对象的净金额进行汇总。有没有其他选择可以使这成为可能？
解决方法
尝试删除第一个查询中的 ToList
var clientPurchases = _context.Purchases
.Where(m => accountIds.Contains(m.AccountId) &&
m.Date >= resultingStartDate &&
m.Date <= resultingEndDate);
var sum = clientPurchases.Sum(m=>m.NetAmount);
说明：在ToList()子句后面链接where之前，没有在sql server上执行过查询。通过添加不带 Sum 的 ToList，您将在 sql server 上计算总和，而无需等待所有符合条件的购买获得 C# 内存空间，然后在 IEnumerable 上计算总和。
在 deferred linq to database execution
上阅读更多相关信息 ,您的 linq 代码生成的 SQL 与您的 SQL 代码不同。您可以使用此 linq 生成正确的 SQL 查询。
var result = (from purchase in dbContext.Purchases
from contract in dbContext.Contracts
join account in dbContext.Accounts on contract.AccountId equals account.Id
into temp
from account in temp.DefaultIfEmpty()
where purchase.Date <= DateTime.Parse("2020-05-08")
&& purchase.Date > DateTime.Parse("2019-05-08")
group purchase by purchase.ClientId
into g
select new
{
ClientId = g.Key,TotalNetAmount = g.Sum(x => x.NetAmount)
}).ToList();
Linq 方法链版本
var result = dbContext.Purchases
.SelectMany(purchase => dbContext.Contracts,(purchase,contract) => new
{
purchase,contract
})
.GroupJoin(dbContext.Accounts,pc => pc.contract.AccountId,account => account.Id,(pc,temp) => new
{
pc,temp
})
.SelectMany(pcTemp => pcTemp.temp.DefaultIfEmpty(),(pcTemp,account) => new
{
pcTemp,account
})
.Where(pcTempAcc => pcTempAcc.pcTemp.pc.purchase.Date <= DateTime.Parse("2020-05-08") &&
pcTempAcc.pcTemp.pc.purchase.Date > DateTime.Parse("2019-05-08"))
.GroupBy(pcTempAcc => pcTempAcc.pcTemp.pc.purchase.ClientId,pcTempAcc => pcTempAcc.pcTemp.pc.purchase)
.Select(g => new
{
ClientId = g.Key,TotalNetAmount = g.Sum(x => x.NetAmount)
}).ToList();
这种方法生成这样的 SQL
SELECT p.client_id AS "ClientId",COALESCE(SUM(p.net_amount),0.0) AS "TotalNetAmount"
FROM "Purchase" AS p
CROSS JOIN "Contract" AS c
LEFT JOIN "Account" AS a ON c.account_id = a."Id"
WHERE (p.purchase_date <= DATE '2020-05-08') AND (p.purchase_date > DATE '2019-05-08')
GROUP BY p.client_id