CODEWITHSUNDEEP
sql-serverentity-frameworkazure-sql-database
Dan
Dan

Reputation: 788

Azure SQL slow performance on parameterized queries NVARCHAR(4000)

I became aware of a performance difference when using parameterized queries generated by EntityFramework when using an Azure SQL database.

I have a table with a varchar(30) primary key and when I try to get a value from that table using the primary key, EntityFramework creates a parameterized query using NVARCHAR(4000) as the datatype.

DECLARE @p__linq__0 nvarchar(4000)
SET @p__linq__0 ='ehenurqp0kpql76kjsw3'

select * from mediacontentreferences where mediacontentreferenceid=@p__linq__0

This generates a very strange execution plan on the Azure SQL database.

Azure SQL execution plan 1

It is quite ineffective since it uses an Index Scan that looks through the whole table.

If I use a parameter with the correct datatype, the execution plan uses the primary key index. 

DECLARE @p__linq__1 varchar(30)
SET @p__linq__1 ='ehenurqp0kpql76kjsw3'

select * from mediacontentreferences where mediacontentreferenceid=@p__linq__1

Azure SQL execution plan 2

If I use the first query on an on-premises SQL server, the execution plan converts the NVARCHAR(4000) to a VARCHAR(30) and uses the primary key index. SQL Server execution plan 1

This looks like a flaw in the execution plan calculation on the Azure SQL server.

Is there a possibility to change the behavior of Entity Framework on how it creates the queries?

I have read a couple articles regarding this but no solution where found there.

Why does code first/EF use 'nvarchar(4000)' for strings in the raw SQL command?

Why does Entity Framework generate large parameters? How can they be reduced?

Upvotes: 3

Views: 768

Answers (1)

Jan Muncinsky
Jan Muncinsky

Reputation: 4408

You must specify right datatype for your column, than EF while generating queries will declare parameters with a right data type. Just add this to your DbContext:

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<mediacontentreferences >().Property(e => e.mediacontentreferenceid).HasColumnType("varchar");
    }

Upvotes: 2

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