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Is there a difference between the following queries, assuming there is a primary field 'id' in the table (as in speed, etc)?
SELECT COUNT(id) FROM table vs.
SELECT COUNT(*) FROM table 
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The simple answer is no – there is no difference at all. The COUNT(*) function counts the total rows in the table, including the NULL values.
The count(*) returns all rows whether column contains null value or not while count(columnName) returns the number of rows except null rows. Let us first create a table.
In this case, COUNT(id) counts the number of rows in which id is not NULL .
MySQL COUNT() Function The COUNT() function returns the number of records returned by a select query.
I know this question is about MySQL, but for what it's worth, count(*) is recommended for Oracle, which goes to show that the answer to this can be database dependent (see comment above from BalusC).
Since a lot of databases (MS-SQL, MySQL) have information schema tables that hold various types of metadata, there are bound to be differences if one syntax is simply looking up a readily-available value, and another is going straight to the table.
At the end of day: try different options, and see what EXPLAIN is telling you is going on behind the scenes.
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