Difference between count(1) and count(*)

Some people prefer count(1) over count(*) and using count(1) to optimize his statements, but actually they have not difference.

Here is test code and data(using the TRAN for execute more speed only):

BEGIN TRAN
CREATE  TABLE TestCount(C1 VARCHAR(400),C2 VARCHAR(300),C3 VARCHAR(150) NOT NULL,C4 VARCHAR(80),C5 VARCHAR(38) NOT NULL,C6 INT)
DECLARE @COUNT INT =1
SET NOCOUNT ON
WHILE @COUNT<=1500000
BEGIN
    INSERT TestCount
        SELECT NEWID(),NEWID(),NEWID(),NEWID(),NEWID(),@COUNT
    SET @COUNT=@COUNT+1
END
commit

As to count(1),count(*),count('anything'), they are the same about results and execultion plan(it used "table scan", because the table is heap)

As to count(column)

It noly count the rows of is not null for the columns:

UPDATE TOP (1) TestCount SET c1 =NULL 

How to execute count(*) of execution engine

Execution engine will use a have index's column to count count(*)(if the table have index), of cause, for performance reasons, engine will prefer the narrowest index.

CREATE NONCLUSTERED INDEX IDX_C5 ON TestCount(C5)

add a index in C6(the column type is int, is 4 bytes)

CREATE NONCLUSTERED INDEX IDX_C6 ON TestCount(C6)

Engine used more  narrow index to count.

 comparison of count(wide index column), count(narrow index column) and count(no index column)

CREATE NONCLUSTERED INDEX IDX_C2 ON TestCount(C2)

So, count(no index column) performance is poor and index the more narrow the better, the performance more batter too.

Now,  we will compare more detail between count(1) and count(*), we already know their "query cost" is same and the table has 3 index: C2/C5/C6

 Scan page is same and  IO cast is same too

SET STATISTICS IO ON
SET STATISTICS TIME ON
SELECT COUNT(1) FROM TestCount    
SET STATISTICS IO OFF
SET STATISTICS TIME OFF
------------------------
PRINT 'NEXT plan'
------------------------
SET STATISTICS IO ON
SET STATISTICS TIME ON
SELECT COUNT(*) FROM TestCount    
SET STATISTICS IO OFF
SET STATISTICS TIME OFF