Question

DBAs have told me that when using T-SQL:

select count(id) from tableName

is faster than

select count(*) from tablenName

if id is the primary key.

Extrapolating that to LINQ-TO-SQL is the following accurate?

This LINQ-to-SQL statement:

int count = dataContext.TableName.Select(primaryKeyId => primaryKeyId).Count();

is more performant than this one:

int count = dataContext.TableName.Count();

Answer 1

As I understand it there’s no difference between your two select count statements.

Using LINQPad we can examine the T-SQL generated by different LINQ statements.

For Linq to SQL both

TableName.Select(primaryKeyId => primaryKeyId).Count();

and

TableName.Count();

generate the same SQL

SELECT COUNT(*) AS [value] FROM [dbo].[TableName] AS [t0]

For Linq to Entites, again they both generate the same SQL, but now it’s

SELECT 
[GroupBy1].[A1] AS [C1]
FROM ( SELECT 
    COUNT(1) AS [A1]
    FROM [dbo].[TableName] AS [Extent1]
)  AS [GroupBy1]