I saw something from an "execution plan" article:
10 rows fetched in 0.0003s (0.7344s)
(the link: http://explainextended.com/2009/09/18/not-in-vs-not-exists-vs-left-join-is-null-mysql/ )
How come there are 2 durations shown? What if I don't have large data set yet. For example, if I have only 20, 50, or even just 100 records, I can't really measure how faster 2 different SQL statements compare in term of speed in real life situation? In other words, there needs to be at least hundreds of thousands of records, or even a million records to accurately compares the performance of those 2 different SQL statements?
For your first question:
X row(s) fetched in Y s (Z s)
X = number of rows (of course); Y = time it took the MySQL server to execute the query (parse, retrieve, send); Z = time the resultset spent in transit from the server to the client;
(Source: http://forums.mysql.com/read.php?108,51989,210628#msg-210628)
For the second question, you will never ever know how the query performs unless you test with a realistic number of records. Here is a good example of how to benchmark correctly: http://www.mysqlperformanceblog.com/2010/04/21/mysql-5-5-4-in-tpcc-like-workload/
That blog in general as well as the book "High Performance MySQL" is a goldmine.