See the query below. The object and property names have been obfuscated somewhat to not leak confidential/sensitive information, but the query structure is the same.
When the .OrderBy(p => "") is added, which is complete non-sense to me, the query runs much faster. The time it takes to execute the query goes from approx. 2000ms down to approx. 400ms. I have tested it a couple of times, adding and removing only the OrderBy statement.
I am completely puzzled, how can this be? The query is executed on a SQL database in an Azure environment.
I can understand that ordering data on property A, and then selecting records where property A equals some value could potentialy speed up the query. But ordering on an empty string!? What is going on here?
Also I want to note, that the query, without the OrderBy, using Expressions ( as suggested in this post to circumvent SQL parameter sniffing) lowers the execution time also to approx. 400ms. Adding the .OrderBy(p => "") then doesn't make any noticeable difference.
var query = (from p in Context.Punders.Where(p => p.A == A)
.Where(p => null != p.SomeNumber)
.Where(p => p.StatusCode == Default ||
p.StatusCode == Cancelled)
.Where(p => p.DatePosted >= startDate && p.DatePosted <= endDate)
join f in Context.Founders.Where(f => f.A == A) on p.Code equals f.Code
join r in Context.Rounders.Where(r => r.A == A) on p.Code equals r.Code
into rg
from r in rg.DefaultIfEmpty()
join pt in Context.FishTypes.Where(ft => ft.A ==A) on p.Code equals pt.Code
where r == null
select new
{
p.Code,
f.B,
f.C,
p.D,
p.E,
pt.F,
pt.G,
p.H
})
.OrderBy(p => "");
Query without the .OrderBy(...
SELECT [Filter1].[q] AS [q],
[Filter1].[c1] AS [edoc],
[Filter1].[oc1] AS [wnrdc],
[Filter1].[otc1] AS [weener],
[Filter1].[ptc1] AS [pmtpdc],
[Extent4].[isr] AS [isr],
[Extent4].[rac] AS [rac],
[Filter1].[arn] AS [arn]
FROM (SELECT [Extent1].[pcid] AS [pcid1],
[Extent1].[edoc] AS [c1],
[Extent1].[pmtpdc] AS [ptc1],
[Extent1].[q] AS [q],
[Extent1].[arn] AS [arn],
[Extent1].[dateposted] AS [DatePosted],
[Extent2].[pcid] AS [pcid2],
[Extent2].[wnrdc] AS [oc1],
[Extent2].[weener] AS [otc1]
FROM [fnish].[post] AS [Extent1]
INNER JOIN [fnish].[olik] AS [Extent2]
ON [Extent1].[olikedoc] = [Extent2].[edoc]
LEFT OUTER JOIN [fnish].[receivable] AS [Extent3]
ON ( [Extent3].[pcid] = @p__linq__4 )
AND ( [Extent1].[edoc] =
[Extent3].[pepstedoc] )
WHERE ( [Extent1].[arn] IS NOT NULL )
AND ( [Extent1].[posttedoc] IN ( N'D', N'X' ) )
AND ( [Extent3].[id] IS NULL )) AS [Filter1]
INNER JOIN [fnish].[paymenttype] AS [Extent4]
ON [Filter1].[ptc1] = [Extent4].[edoc]
WHERE ( [Filter1].[pcid1] = @p__linq__0 )
AND ( [Filter1].[dateposted] >= @p__linq__1 )
AND ( [Filter1].[dateposted] <= @p__linq__2 )
AND ( [Filter1].[pcid2] = @p__linq__3 )
AND ( [Extent4].[pcid] = @p__linq__5 )
Query with the .OrderBy(...
SELECT [Project1].[q] AS [q],
[Project1].[edoc] AS [edoc],
[Project1].[wnrdc] AS [wnrdc],
[Project1].[weener] AS [weener],
[Project1].[pmtpdc] AS [pmtpdc],
[Project1].[isr] AS [isr],
[Project1].[rac] AS [rac],
[Project1].[arn] AS [arn]
FROM (SELECT N'' AS [C1],
[Filter1].[c1] AS [edoc],
[Filter1].[ptc1] AS [pmtpdc],
[Filter1].[q] AS [q],
[Filter1].[arn] AS [arn],
[Filter1].[oc1] AS [wnrdc],
[Filter1].[otc1] AS [weener],
[Extent4].[isr] AS [isr],
[Extent4].[rac] AS [rac]
FROM (SELECT [Extent1].[pcid] AS [pcid1],
[Extent1].[edoc] AS [c1],
[Extent1].[pmtpdc] AS [ptc1],
[Extent1].[q] AS [q],
[Extent1].[arn] AS [arn],
[Extent1].[dateposted] AS [DatePosted],
[Extent2].[pcid] AS [pcid2],
[Extent2].[wnrdc] AS [oc1],
[Extent2].[weener] AS [otc1]
FROM [fnish].[post] AS [Extent1]
INNER JOIN [fnish].[olik] AS [Extent2]
ON [Extent1].[olikedoc] = [Extent2].[edoc]
LEFT OUTER JOIN [fnish].[receivable] AS [Extent3]
ON ( [Extent3].[pcid] =
@p__linq__4 )
AND ( [Extent1].[edoc] =
[Extent3].[pepstedoc] )
WHERE ( [Extent1].[arn] IS NOT NULL )
AND ( [Extent1].[posttedoc] IN ( N'D', N'X' ) )
AND ( [Extent3].[id] IS NULL )) AS [Filter1]
INNER JOIN [fnish].[paymenttype] AS [Extent4]
ON [Filter1].[ptc1] = [Extent4].[edoc]
WHERE ( [Filter1].[pcid1] = @p__linq__0 )
AND ( [Filter1].[dateposted] >= @p__linq__1 )
AND ( [Filter1].[dateposted] <= @p__linq__2 )
AND ( [Filter1].[pcid2] = @p__linq__3 )
AND ( [Extent4].[pcid] = @p__linq__5 )) AS [Project1]
ORDER BY [Project1].[c1] ASC
Conclusion
From what I have learned, with a bit of a guess: It is case specific behavior. In my case, the performance gain is likely due to a different execution plan being constructed by the SQL server that is yielding a better performing query. I've seen a different execution plan with the query without the OrderBy using the SQL statement OPTION(RECOMIPILE) that showed similar performance gain. So adding the OrderBy to the LINQ query is very likely (I think) producing a different execution plan that yields a better performing query.
