Why does this EF query produce such crazy generated SQL? - sql

Question

var statusId = db.WorkOrder.Where(w => w.OrderType.Name == "ReadAudit" && w.WorkOrderMapping.MeterOldTag == meterTag && w.OrderStatusId != 80)
.OrderByDescending(w => w.CreationDatetime)
.Select(r => r.OrderStatusId)
.FirstOrDefault();
That produces this crazy sql:
SELECT TOP (1)
[Project1].[OrderStatusId] AS [OrderStatusId]
FROM ( SELECT
[Filter1].[OrderStatusId] AS [OrderStatusId],
[Filter1].[CreationDatetime] AS [CreationDatetime]
FROM (
SELECT [Extent1].[OrderStatusId] AS [OrderStatusId],
[Extent1].[CreationDatetime] AS [CreationDatetime],
[Extent3].[MeterOldTag] AS [MeterOldTag]
FROM [dbo].[WorkOrder] AS [Extent1]
INNER JOIN [dbo].[OrderType] AS [Extent2] ON [Extent1].[OrderTypeKey] = [Extent2].[OrderTypeKey]
LEFT OUTER JOIN [dbo].[WorkOrderMapping] AS [Extent3] ON [Extent1].[WorkOrderKey] = [Extent3].[WorkOrderMappingKey]
WHERE (80 <> [Extent1].[OrderStatusId])
AND (N'ReadAudit' = [Extent2].[Name])
) AS [Filter1]
WHERE ([Filter1].[MeterOldTag] = #p__linq__0) OR (([Filter1].[MeterOldTag] IS NULL) AND (#p__linq__0 IS NULL))
) AS [Project1]
ORDER BY [Project1].[CreationDatetime] DESC
And I'm told that its hitting the database pretty hard:
Table 'WorkOrder'. Scan count 30, logical reads 84403
Table 'WorkOrderMapping'. Scan count 9, logical reads 16516
The EF query doesn't seem that complicated. Is there a way to get the generated SQL to be more efficient?

Answer 1

The only thing "crazy" about that SQL Query is the predicate on MeterOldTag. It's written like that because EF, by default, writes queries to emulate C# comparison semantics for LINQ queries. If you want a simple equality comparison in the databse, set UseDatabaseNullSemantics for your DbContext.

Answer 2

It is just how Entity Framework works.
If you want more control, you can write the query yourself via Entity Framework Raw Queries, see here and below a (not complete) example how your query might look like.
DbRawSqlQuery<Int32> query = db.Database.SqlQuery<Int32>("SELECT OrderStatusId FROM ... ");
var statusId = query.FirstOrDefault();
Edit:
Also have a look at the actual query plan that gets executed for this query in order to find if whether the appropriate indexes are present on the tables involved.
Consider moving the constants RealAuditand 80 to variables, like below:
var orderType = "RealAudit";
var orderStatusId = 80;
var statusId = db.WorkOrder.Where(w => w.OrderType.Name == orderType
&& w.WorkOrderMapping.MeterOldTag == meterTag
&& w.OrderStatusId != orderStatusId
)
.OrderByDescending(w => w.CreationDatetime)
.Select(r => r.OrderStatusId)
.FirstOrDefault();
By doing so, these will appear as SQL parameters in the query, something like:
#p__linq__1 <> [Extent1].[OrderStatusId])
AND (#p__linq__2 = [Extent2].[Name])
This allows that a single query plan can be used for all variations of this query, whereas now you get a queryplan per separate value of the the MeterOldTag argument.