I came across an issue with EF creating terrible queries when searching on a string field. Its produced a query in the style of lazy programmers to encompass null checking which forces the whole index to be scanned.
consider the following queries.
Query 1
var p1 = "x"; var r1 = ctx.Set<E>().FirstOrDefault( subject => p1.Equals(subject.StringField));
Query 2
const string p2 = "x"; var r2 = ctx.Set<E>().FirstOrDefault( subject => p2.Equals(subject.StringField));
Query 1 produces
WHERE (('x' = [Extent2].[StringField]) OR (('x' IS NULL) AND ([Extent2].[StringField] IS NULL)))
and executes in 4 seconds
Query 2 produces
WHERE (N'x' = [Extent2].[StringField])
and executes in 2 milliseconds
Does anyone know of any work arounds? (no the parameter cant be a const as it is entered by user input but cannot be null.)
N.B When profiled, both queries are prepared with sp_executesql by EF; as of-cause if they were just executed the query optimiser would negate the OR 'x' IS NULL check.
To view the SQL that will be generated, simply call ToTraceString() . You can add it into your watch window and set a breakpoint to see what the query would be at any given point for any LINQ query.
Data Annotations - Required Attribute in EF 6 & EF CoreEF will create a NOT NULL column in a database table for a property on which the Required attribute is applied.
Set UseDatabaseNullSemantics = true
;
When UseDatabaseNullSemantics == true
, (operand1 == operand2)
will be translated as:
WHERE operand1 = operand2
When UseDatabaseNullSemantics == false
, (operand1 == operand2)
will be translated as:
WHERE ( (operand1 = operand2) AND (NOT (operand1 IS NULL OR operand2 IS NULL)) ) OR ( (operand1 IS NULL) AND (operand2 IS NULL) )
This is documented by Microsoft:
Gets or sets a value indicating whether database null semantics are exhibited when comparing two operands, both of which are potentially nullable. The default value is false.
You can set it in your DbContext
subclass constructor, like so:
public class MyContext : DbContext { public MyContext() { this.Configuration.UseDatabaseNullSemantics = true; } }
Or you can also set this setting to your dbContext
instance from the outside like the code example below, from my point of view (see @GertArnold comment), this apporach will be better, because it will not change the default database behaviour or configuration):
myDbContext.Configuration.UseDatabaseNullSemantics = true;
You can fix this by adding [Required]
on StringField property
public class Test { [Key] public int Id { get; set; } [Required] public string Bar{ get; set; } public string Foo { get; set; } } string p1 = "x"; var query1 = new Context().Tests.Where(F => p1.Equals(F.Bar)); var query2 = new Context().Tests.Where(F => p1.Equals(F.Foo));
this is query1
{SELECT [Extent1].[Id] AS [Id], [Extent1].[Bar] AS [Bar], [Extent1].[Foo] AS [Foo] FROM [dbo].[Tests] AS [Extent1] WHERE @p__linq__0 = [Extent1].[Bar]}
and this is query2
{SELECT [Extent1].[Id] AS [Id], [Extent1].[Bar] AS [Bar], [Extent1].[Foo] AS [Foo] FROM [dbo].[Tests] AS [Extent1] WHERE (@p__linq__0 = [Extent1].[Foo]) OR ((@p__linq__0 IS NULL) AND ([Extent1].[Bar2] IS NULL))}
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