Re: Rows Examined, why so many?

[Daniel Nichter]

http://hackmysql.com/joinrows is my understanding of this. That is, it has to do with how MySQL joins tables, which causes the number of rows examined (i.e., read), to be different from the actual number of rows produced (i.e., returned). If you have a dead quiet test server, or are using MySQL 5.0.2 or higher, I suggest:
1. FLUSH STATUS;
2. Run the query
3. SHOW STATUS LIKE 'Handler_read%';

The Handler_read_* values give good insight into what kind of activity MySQL runs itself through while doing a join. Sometimes, through these values you can trace exactly what MySQL does (knowing first how it joins tables, and then not always because it has optimizations which allow it to work less).