SQL Server'da, bir dizin araması yalnızca kümelenmiş bir dizine sahip bir masaya zorlandığında açıkça bir ORDER BY deyimi olmadan bir sipariş garanti edebilir miyim?

Güncelleme 2014-12-18

Asıl soruya “Hayır” olarak verilen ezici yanıtla, daha ilginç olan cevaplar, performans bulmacasını açık bir şekilde nasıl çözeceğimiz konusunda 2. bölüme odaklandı ORDER BY. Şimdiden bir cevabı işaretlememe rağmen, daha iyi performans gösteren bir çözüm olsaydı şaşırmam.

orijinal

Bu soru ortaya çıktı, çünkü belirli bir soruna bulabildiğim son derece hızlı çözüm, sadece bir ORDER BYmadde olmadan işe yaradı. Aşağıda, sorunumu ortaya çıkarmak için gereken tam T-SQL, önerilen çözümümle birlikte (Önemliyse, SQL Server 2008 R2 kullanıyorum.)

--Create Orders table
IF OBJECT_ID('tempdb..#Orders') IS NOT NULL DROP TABLE #Orders
CREATE TABLE #Orders
(  
       OrderID    INT NOT NULL IDENTITY(1,1)
     , CustID     INT NOT NULL
     , StoreID    INT NOT NULL       
     , Amount     FLOAT NOT NULL
)
CREATE CLUSTERED INDEX IX ON #Orders (StoreID, Amount DESC, CustID)

--Add 1 million rows w/ 100K Customers each of whom had 10 orders
;WITH  
    Cte0 AS (SELECT 1 AS C UNION ALL SELECT 1), --2 rows  
    Cte1 AS (SELECT 1 AS C FROM Cte0 AS A, Cte0 AS B),--4 rows  
    Cte2 AS (SELECT 1 AS C FROM Cte1 AS A ,Cte1 AS B),--16 rows 
    Cte3 AS (SELECT 1 AS C FROM Cte2 AS A ,Cte2 AS B),--256 rows 
    Cte4 AS (SELECT 1 AS C FROM Cte3 AS A ,Cte3 AS B),--65536 rows 
    Cte5 AS (SELECT 1 AS C FROM Cte4 AS A ,Cte2 AS B),--1048576 rows 
    FinalCte AS (SELECT  ROW_NUMBER() OVER (ORDER BY C) AS Number FROM   Cte5)
INSERT INTO #Orders (CustID, StoreID, Amount)
SELECT CustID = Number / 10
     , StoreID    = Number % 4
     , Amount     = 1000 * RAND(Number)
FROM  FinalCte
WHERE Number <= 1000000

SET STATISTICS IO ON
SET STATISTICS TIME ON

--For StoreID = 1, find the top 500 customers ordered by their most expensive purchase (Amount)

--Solution A: Without ORDER BY
DECLARE @Top INT = 500
SELECT DISTINCT TOP (@Top) CustID
FROM #Orders WITH(FORCESEEK)
WHERE StoreID = 1
OPTION(OPTIMIZE FOR (@Top = 1), FAST 1);
--9 logical reads, CPU Time = 0 ms, elapsed time = 1 ms
GO
--Solution B: With ORDER BY
DECLARE @Top INT = 500
SELECT TOP (@Top) CustID
FROM #Orders
WHERE StoreID = 1
GROUP BY CustID
ORDER BY MAX(Amount) DESC
OPTION(MAXDOP 1)
--745 logical reads, CPU Time = 141 ms, elapsed time = 145 ms
--Uses Sort operator

GO

Çözüm A ve B için sırasıyla yürütme planları:

Çözüm A, ihtiyacım olan performansı veriyor, ancak herhangi bir SİPARİŞ BY cümlesi eklerken aynı performansla çalışmasını sağlayamadım (örneğin, bkz. Çözüm B). Ve kesinlikle Çözüm A'nın sonuçlarını sırayla vermek zorunda kalacak gibi görünüyor, çünkü 1) tablonun üzerinde tek bir dizin var, 2) bir arama zorunluluğu var, böylece IAM sayfalarına dayalı bir tahsisat siparişi taraması kullanma olasılığını ortadan kaldırıyor .

Yani benim sorularım:

1. Bu durumda emri fıkra emri olmadan vermeyi garanti edeceği doğru muyum?

2. Olmazsa, Çözüm A kadar hızlı bir planı zorlamak için başka bir yöntem var mı, tercihen çeşitlerden kaçınır? Aynı sorunu çözmek zorunda kalacağını unutmayın (çünkü StoreID = 1en pahalı satın alma tutarlarına göre sipariş edilen ilk 500 müşteriyi bulmak için). Yine de #Orderstabloyu kullanmak zorunda kalacaktı , ancak farklı indeksleme şemaları uygun olacaktır.

1. Bu durumda emri fıkra emri olmadan vermeyi garanti edeceği doğru muyum?

Hayır . SQL Server'da düzeni koruyan ( sıralama yapılmadan izin veren ) bir Akış AyrıntısıORDER BY uygulanmadı. Prensip olarak yapmak mümkündür, ancak SQL Server kaynak kodunu değiştirmemize izin verilirse birçok şey mümkündür. Bu geliştirme çalışması için iyi bir örnek yapabilirseniz , bunu Microsoft'a önerebilirsiniz .

2. Olmazsa, Çözüm A kadar hızlı bir planı zorlamak için başka bir yöntem var mı, tercihen çeşitlerden kaçınır?

Evet. (Tablo ve sorgu ipuçları yalnızca 2014 öncesi kardinalite tahmincisini kullanırken gerekli):

-- Additional index
CREATE UNIQUE NONCLUSTERED INDEX i 
ON #Orders (StoreID, CustID, Amount, OrderID);

-- Query
SELECT TOP (500) 
    O.CustID, 
    O.Amount
FROM #Orders AS O
    WITH (FORCESEEK(IX (StoreID)))
WHERE O.StoreID = 1
AND NOT EXISTS
(
    SELECT NULL
    FROM #Orders AS O2
        WITH (FORCESEEK(i (StoreID, CustID, Amount)))
    WHERE 
        O2.StoreID = O.StoreID
        AND O2.CustID = O.CustID
        AND O2.Amount >= O.Amount
        AND
        (
            O2.Amount > O.Amount
            OR
            (
                O2.Amount = O.Amount
                AND O2.OrderID > O.OrderID
            )
        )
)
ORDER BY
    O.Amount DESC
OPTION (MAXDOP 1);

(500 row(s) affected)

 SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 4 ms.

SQL CLR çözümü

Aşağıdaki komut, belirtilen gereksinimleri karşılamak için SQL CLR tablo değerli işlevini kullanarak gösterir. Ben bir C # uzmanı değilim, bu yüzden kod geliştirme yapabilir:

USE Sandpit;
GO
-- Ensure SQLCLR is enabled
EXECUTE sys.sp_configure
    @configname = 'clr enabled',
    @configvalue = 1;
RECONFIGURE;
GO
-- Lazy, but effective to allow EXTERNAL_ACCESS
ALTER DATABASE Sandpit
SET TRUSTWORTHY ON;
GO
-- The CLR assembly
CREATE ASSEMBLY FlowDistinctOrder
AUTHORIZATION dbo
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WITH PERMISSION_SET = EXTERNAL_ACCESS;
GO
-- The CLR TVF with order guarantee
CREATE FUNCTION dbo.FlowDistinctOrder 
(
    @ServerName nvarchar(128), 
    @DatabaseName nvarchar(128), 
    @MaxRows bigint
)
RETURNS TABLE 
(
    CustID integer NULL, 
    Amount float NULL
)
ORDER (Amount DESC)
AS EXTERNAL NAME FlowDistinctOrder.UserDefinedFunctions.FlowDistinctOrder;

Test tablosu ve sorudan örnek veriler:

-- Test table
CREATE TABLE dbo.Orders
(  
    OrderID    integer  NOT NULL IDENTITY(1,1),
    CustID     integer  NOT NULL,
    StoreID    integer  NOT NULL,
    Amount     float    NOT NULL
);
GO
-- Sample data
WITH  
    Cte0 AS (SELECT 1 AS C UNION ALL SELECT 1), --2 rows  
    Cte1 AS (SELECT 1 AS C FROM Cte0 AS A, Cte0 AS B),--4 rows  
    Cte2 AS (SELECT 1 AS C FROM Cte1 AS A ,Cte1 AS B),--16 rows 
    Cte3 AS (SELECT 1 AS C FROM Cte2 AS A ,Cte2 AS B),--256 rows 
    Cte4 AS (SELECT 1 AS C FROM Cte3 AS A ,Cte3 AS B),--65536 rows 
    Cte5 AS (SELECT 1 AS C FROM Cte4 AS A ,Cte2 AS B),--1048576 rows 
    FinalCte AS (SELECT  ROW_NUMBER() OVER (ORDER BY C) AS Number FROM   Cte5)
INSERT dbo.Orders 
    (CustID, StoreID, Amount)
SELECT 
    CustID  = Number / 10,
    StoreID = Number % 4,
    Amount  = 1000 * RAND(Number)
FROM FinalCte
WHERE 
    Number <= 1000000;
GO
-- Index
CREATE CLUSTERED INDEX IX 
ON dbo.Orders 
    (StoreID ASC, Amount DESC, CustID ASC);

Fonksiyon testi:

-- Test the function
-- Run several times to ensure connection is cached
-- and CLR code fully compiled
DECLARE @Start datetime2 = SYSUTCDATETIME();

SELECT TOP (500) 
    FDO.CustID
FROM dbo.FlowDistinctOrder
(
    @@SERVERNAME,   -- For external connection
    DB_NAME(),      -- For external connection
    500             -- Number of rows to return
) AS FDO 
ORDER BY 
    FDO.Amount DESC;

SELECT DATEDIFF(MILLISECOND, @Start, SYSUTCDATETIME());

Uygulama planı ( ORDERgarantinin onaylandığına dikkat edin ):

Dizüstü bilgisayarımda, bu genellikle 80-100ms'de çalışır. Bu, yukarıda T-SQL kadar tekrar yazmak kadar hızlı değildir, ancak farklı veri dağılımları karşısında iyi performans kararlılığı göstermesi gerekir.

Kaynak kodu:

using Microsoft.SqlServer.Server;
using System.Collections;
using System.Collections.Generic;
using System.Data.SqlClient;

public partial class UserDefinedFunctions
{
    private sealed class ReverseComparer<T> : IComparer<T>
    {
        private readonly IComparer<T> original;

        public ReverseComparer(IComparer<T> original)
        {
            this.original = original;
        }

        public int Compare(T left, T right)
        {
            return original.Compare(right, left);
        }
    }

    [SqlFunction
        (
        DataAccess = DataAccessKind.Read,
        SystemDataAccess = SystemDataAccessKind.None,
        FillRowMethodName = "FillRow",
        TableDefinition = "CustID integer NULL, Amount float NULL"
        )
    ]
    public static IEnumerable FlowDistinctOrder
        (
        [SqlFacet (MaxSize=128)]string ServerName, 
        [SqlFacet (MaxSize=128)]string DatabaseName,
        long MaxRows
        )
    {
        var list = new SortedDictionary<double, int>
            (new ReverseComparer<double>(Comparer<double>.Default));

        var csb = new SqlConnectionStringBuilder();
        csb.ConnectTimeout = 10;
        csb.DataSource = ServerName;
        csb.Enlist = false;
        csb.InitialCatalog = DatabaseName;
        csb.IntegratedSecurity = true;

        using (var conn = new SqlConnection(csb.ConnectionString))
        {
            conn.Open();
            using (var cmd = conn.CreateCommand())
            {
                cmd.CommandText =
                    @"
                    SELECT
                        O.CustID, 
                        O.Amount
                    FROM dbo.Orders AS O
                    WHERE 
                        O.StoreID = 1 
                    ORDER BY 
                        O.Amount DESC";

                int custid;
                double amount;

                using (var rdr = cmd.ExecuteReader())
                {
                    while (rdr.Read())
                    {
                        custid = rdr.GetInt32(0);
                        amount = rdr.GetDouble(1);

                        if (!list.ContainsKey(amount))
                        {
                            list.Add(amount, custid);
                            if (list.Count == MaxRows)
                            {
                                break;
                            }
                        }
                    }
                }
            }
        }
        return list;
    }

    public static void FillRow(object obj, out int CustID, out double Amount)
    {
        var v = (KeyValuePair<double, int>)obj;
        CustID = v.Value;
        Amount = v.Key;
    }
}

ORDER BYÇok fazla şey olmadan yanlış gidebilir. Aklıma gelebilecek tüm olası sorunları hariç tuttunuz, ancak bu bir sorun çıkmayacağı ve gelecek sürümlerde bir sorun olmayacağı anlamına gelmez.

Bu çalışmalı:

Bir döngü içinde masadan 500 satırlık gruplar çekin ve 500 farklı müşteri kimliğiniz olduğunda durun. Getirme sorgusu şöyle görünebilir:

select TOP (500) Amount, CustID
into #fetchedOrders
from Orders
where StoreID = 1234 and Amount <= @lastAmountFetched
order by Amount DESC

Bu, dizinde sıralı aralık taraması gerçekleştirir. Amount <= @lastAmountFetchedYüklem aşamalı fazla kayıt yok çıkarmaktır. Her sorgu yalnızca fiziksel olarak 500 kayda dokunacak. Bu, O (1) olduğu anlamına gelir. Endekse girdiğiniz alan daha pahalı hale gelmiyor.

Bu @lastAmountFetchedifadede aldığınız en küçük değere düşmek için değişkeni korumalısınız .

Bu şekilde dizini aşamalı olarak düzenli bir şekilde tarayacaksınız. En fazla (500 - 1) satır olacak en uygun miktardan daha fazla okuyacaksınız.

Bu, belirli bir mağaza için her zaman 100000 veya daha fazla siparişin toplanmasından çok daha hızlı olacaktır. Muhtemelen, her biri sadece 500 satırlık birkaç tekrarlamaya ihtiyaç duyulacaktır.

Temel olarak, bu manuel olarak kodlanmış bir akış özelliği olan bir operatördür.

Alternatif olarak, mümkün olduğunca az satır almak için bir imleç kullanın. Bu, 500 tek sıralı sorguyu yürütme işleminin genellikle 500 satırlık bir toplu iş yürütme işleminden daha yavaş olduğu için çok daha yavaş olacaktır.

Alternatif olarak, DISTINCTsıralı bir şekilde tüm satırları sorgula ve yeterli sayıda satır döndürüldüğünde (kullanarak SqlCommand.Cancel) istemci uygulamasının sorguyu sonlandırmasını sağla .