Question

In MySQL, is it generally a good idea to always do a COUNT(*) first to determine if you should do a SELECT * to actually fetch the rows, or is it better to just do the SELECT * directly and then check if it returned any rows?

Answer 1

Unless you lock the table/s in question, doing a select count(*) is useless. Consider:

Process 1:

SELECT COUNT(*) FROM T;

Process 2:

INSERT INTO T

Process 1:

...now doing something based on the obsolete count retrieved before...

Of course, locking a table is not a very good idea in a server environment.