In SQL Server, posso garantire un ordine senza una clausola ORDER BY esplicita quando una ricerca dell'indice è forzata su una tabella con solo un indice cluster?

Aggiornamento 18/12/2014

Poiché la risposta schiacciante alla domanda principale è "No", le risposte più interessanti si sono concentrate sulla parte 2, su come risolvere il puzzle delle prestazioni in modo esplicito ORDER BY. Anche se ho già segnato una risposta, non sarei sorpreso se ci fosse una soluzione ancora migliore.

Originale

Questa domanda è nata perché l'unica soluzione estremamente veloce che ho potuto trovare per un particolare problema funziona solo senza una ORDER BYclausola. Di seguito è riportato il T-SQL completo necessario per produrre il problema, insieme alla mia soluzione proposta (sto usando SQL Server 2008 R2, se è importante).

--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

Ecco i piani di esecuzione per la soluzione A e B, rispettivamente:

La soluzione A offre le prestazioni di cui ho bisogno, ma non sono riuscito a farlo funzionare con le stesse prestazioni quando si aggiunge una clausola ORDER BY di qualsiasi tipo (ad esempio, vedere la soluzione B). E certamente sembra che la soluzione A dovrebbe fornire i suoi risultati in ordine, poiché 1) la tabella ha un solo indice su di essa, 2) viene forzata una ricerca, eliminando così la possibilità di utilizzare una scansione dell'ordine di allocazione basata su pagine IAM .

Quindi le mie domande sono:

1. Ho ragione che garantirà l'ordine in questo caso senza una clausola order by?

2. In caso contrario, esiste un altro metodo per forzare un piano più veloce della soluzione A, preferibilmente uno che evita le specie? Si noti che dovrebbe risolvere esattamente lo stesso problema (per StoreID = 1, trovare i primi 500 clienti ordinati in base all'importo di acquisto più costoso). Dovrebbe anche usare ancora la #Orderstabella, ma diversi schemi di indicizzazione andrebbero bene.

1. Ho ragione che garantirà l'ordine in questo caso senza una clausola order by?

No. Un Flow Distinct che conserva l'ordine (consentendo ORDER BYsenza ordinamento) non è implementato in SQL Server oggi. È possibile fare in linea di principio, ma poi molte cose sono possibili se ci è permesso cambiare il codice sorgente di SQL Server. Se riesci a fare un buon caso per questo lavoro di sviluppo, potresti suggerirlo a Microsoft .

2. In caso contrario, esiste un altro metodo per forzare un piano più veloce della soluzione A, preferibilmente uno che evita le specie?

Sì. (Suggerimenti per tabelle e query richiesti solo quando si utilizza lo stimatore di cardinalità pre-2014):

-- 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.

Soluzione CLR SQL

Lo script seguente mostra l'utilizzo di una funzione con valori di tabella CLR SQL per soddisfare i requisiti indicati. Non sono un esperto di C #, quindi il codice potrebbe apportare miglioramenti:

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
FROM 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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;

Tabella di prova e dati di esempio dalla domanda:

-- 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);

Test di funzionalita:

-- 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());

Piano di esecuzione (notare la convalida della ORDERgaranzia):

Sul mio laptop, questo in genere viene eseguito in 80-100ms. Questo non è affatto veloce come la riscrittura T-SQL sopra, ma dovrebbe mostrare una buona stabilità delle prestazioni di fronte alle diverse distribuzioni di dati.

Codice sorgente:

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;
    }
}

Senza ORDER BYmolte cose può andare storto. Hai escluso tutti i possibili problemi che mi vengono in mente, ma ciò non significa che non ci siano problemi né ce ne sarà uno in una versione futura.

Questo dovrebbe funzionare:

Estrai i batch di 500 righe dalla tabella in un ciclo e fermati quando hai 500 ID cliente distinti. La query di recupero potrebbe essere simile al seguente:

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

Ciò eseguirà una scansione dell'intervallo ordinato sull'indice. Il Amount <= @lastAmountFetchedpredicato è lì per estrarre incrementalmente più record. Ogni query toccherà solo fisicamente 500 record. Ciò significa che è O (1). Non diventa più costoso quanto più si arriva all'indice.

Devi mantenere la variabile @lastAmountFetchedper ridurla al valore più piccolo che hai recuperato in quell'istruzione.

In questo modo scansionerai progressivamente l'indice in modo ordinato. Leggerai al massimo (500 - 1) righe in più rispetto a quanto sarebbe stato ottimale.

Questo sarà molto più veloce di sempre aggregando circa 100000 ordini per un particolare negozio. Probabilmente saranno necessarie solo poche iterazioni di 500 righe ciascuna.

In sostanza, si tratta di un operatore distinto di flusso codificato manualmente.

In alternativa, utilizzare un cursore per recuperare il minor numero di righe possibile. Questo sarà molto più lento perché l'esecuzione di 500 query a riga singola il più delle volte è più lenta dell'esecuzione di un batch di 500 righe.

In alternativa, è sufficiente DISTINCTeseguire una query su tutte le righe senza in un modo ordinato e fare in modo che l'applicazione client termini la query una volta che sono state restituite abbastanza righe (utilizzando SqlCommand.Cancel).