In SQL Server, can I guarantee an order without an explicit ORDER BY clause when an index seek is forced on a table with only a clustered index?
- Am I right that it will guarantee the order in this case without an order by clause?
No. A Flow Distinct that preserves order (allowing ORDER BY without a sort) is not implemented in SQL Server today. It is possible to do in principle, but then many things are possible if we are allowed to change the SQL Server source code. If you can make a good case for this development work, you could suggest it to Microsoft.
- If not, is there another method to force a plan that is as fast as Solution A, preferably one that avoids sorts?
Yes. (Table & query hints only required when using the pre-2014 cardinality estimator):
-- 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 solution
The following script shows using a SQL CLR table-valued function to meet the stated requirements. I am not a C# expert, so the code may bear improvement:
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;
Test table and sample data from the question:
-- 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);
Function test:
-- 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());
Execution plan (note the validation of the ORDER guarantee):
On my laptop, this typically executes in 80-100ms. This is nowhere near as fast as the T-SQL rewrite above, but it should show good performance stability in the face of different data distributions.
Source code:
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;
}
}
Without an ORDER BY a lot of things can go wrong. You have excluded all possible problems that I can think of, but that does not mean that there is no problem nor will there be one in a future release.
This should work:
Pull batches of 500 rows from the table in a loop and stop when you've got 500 distinct customer IDs. The fetch query could look like this:
select TOP (500) Amount, CustID
into #fetchedOrders
from Orders
where StoreID = 1234 and Amount <= @lastAmountFetched
order by Amount DESC
This will perform an ordered range scan on the index. The Amount <= @lastAmountFetched predicate is there to incrementally pull more records. Each query will only physically touch 500 records. That means it is O(1). It does not become more expensive the farther you get into the index.
You have to maintain the variable @lastAmountFetched to decrease to the smallest value that you fetched in that statement.
This way you will incrementally scan the index in an ordered way. You will read at most (500 - 1) rows more than the optimal amount would have been.
This will be a lot faster than always aggregating 100000 or so orders for a particular store. Probably, only a few iterations of 500 rows each will be needed.
Essentially, this is a manually coded flow distinct operator.
Alternatively, use a cursor to fetch as few rows as possible. This will be a lot slower because executing 500 single-row queries most often is slower than executing a batch of 500 rows.
Alternatively, simply query all rows without DISTINCT in an ordered way and make the client application terminate the query once enough rows have been returned (using SqlCommand.Cancel).