如果在 Oracle 中传递参数，实体框架查询会变慢

Question

Here is my query which take about ~15 seconds to execute. The table has an index on REQID

var tls = (from tradelines in _context.TRADELINEs
    where tradelines.REQID == requestId 
    select tradelines).ToList();

Model:

[StringLength(36)]
[Column(TypeName = "char")]
public string REQID { get; set; }

Oracle Db Type for REQID is CHAR(36 BYTE)

If I execute the same query with hard coded string it return the data in ~1 sec.

var tls = (from tradelines in _context.TRADELINEs
    where tradelines.REQID == "2435d403-0beb-416f-a536-0df4e2fcc03f"
    select tradelines).ToList();

I have already tried this approach:

EntityFunctions.AsNonUnicode(requestId)

Still no luck. Any idea what I am missing here?

Doing this :

[StringLength(36)]
[Column(TypeName = "varchar")]
public string REQID { get; set; }

throws an error saying

The store type 'varchar' could not be found in the OracleEFProvider provider manifest

Also, there is no change in execution time if I add

[Column(TypeName = "varchar2")]

EF Log Query:

SELECT
    // Column List
FROM "TRADELINE" "Extent1"
WHERE (("Extent1"."REQID" = :p__linq__0) OR (("Extent1"."REQID" IS NULL) AND (:p__linq__0 IS NULL)))

-- p__linq__0: '2435d403-0beb-416f-a536-0df4e2fcc03f' (Type = Object)

I am wondering why it is generating (Type = Object) for string parameter?

Now, this is interesting:

There is another column in same table with following structure,

[StringLength(36)]
[Column(TypeName = "char")]
public string ID { get; set; }

This column is having primary key with the datatype CHAR(36 BYTE) . If I apply the where condition on this column and specify parameter value, it take only few milliseconds to execute. The only difference between ID and REQID is REQID is allow null.

Answer 1

Taken from this answer ...

public class MyContext : DbContext
{
    public MyContext()
    {
        this.Configuration.UseDatabaseNullSemantics = true;
    }
}