Question : count(*) vs count(0)

Is there any performance gains by using count(0) vs count(*) when getting the total row count of a table?
Someone suggested count(0) is faster because it doesn't count all the columns in the table only the first column.

Answer : count(*) vs count(0)

Many people believe COUNT(columnname) is faster than using COUNT(*), because COUNT(*) would have to read all columns of each row (just like executing a SELECT * FROM MYTABLE statement), while COUNT(columnname) only need to read the specified column. This is not true though, for several reasons.

SQL Server can't read just the contents of a single column without reading the entire row. SQL Server stores the rows with the data on 8 KB data pages on disk. These pages contain one or more rows (depending on the size of each individual row, which may be up to 8060 bytes, with some exceptions), and these pages are placed in the internal memory (RAM) when SQL Server needs to access them for any reason. To check the value of a single column (or several of course), an entire page has to be read from disk and placed in memory. The pages may of course already be cached in memory, in which case the read will be much faster, but SQL still needs to read an entire page from memory just to check a single column of a row.

Now, to avoid having to read these entire data pages when all you are really interested in is how many rows there are in a table, SQL Server will use an index instead, if one exists. Indexes are stored in the same way as data, on 8 KB index pages. Since an index is probably not as wide as a data row (the index only consists of some or even one of the columns in the row), an index page can usually fit a lot more rows per page than the data pages can. This means that SQL Server doesn't have to read as many pages to check the number of rows in the index as it does with the data pages, which is of course a good thing.

This does not only apply to COUNT(columnname_with_an_index_defined_on_it), COUNT(*) will of course also use the index to count the rows. In some cases there may not be an index that covers the specified column in a COUNT(columnname) query, but there is an index defined on another column of the table. In this case COUNT(*) would use this other index to count the number of rows, but COUNT(columnname_without_an_index) would have to read the data pages to check the column for NULL values and count the rows.