In many case we need to use COUNT function, but always we prefer COUNT(column_name) in place
of COUNT(*) and reason behind that COUNT(column_name) will perform better or faster than COUNT(*).
Is this correct?
SQL Server can't read just the contents of a single column without reading the entire row.
So that if we use COUNT(column_name) then also SQL needs to read an entire page from memory just to check a single column of a row.
Let we discuss another case.
We use COUNT(column_name) to count the number of rows of a table where column name is a column that does not allow NULL values.
Now suppose after some time definition of this column gets changed and it is allowing null values, so now column may contain null values.
We may not expect the above situation, and it could possibly cause major problems.
Now try this
SET STATISTICS IO ON
SELECT COUNT(*) FROM Table_Name
SELECT COUNT(column_name) FROM Table_Name
SET STATISTICS IO OFF
when you see the result and read messages you are also agree with my opinion.
