Description:
http://dev.mysql.com/doc/refman/5.0/en/distinct-optimization.html says:
"If you do not use columns from all tables named in a query, MySQL stops scanning any
unused tables as soon as it finds the first match. In the following case, assuming that
t1 is used before t2 (which you can check with EXPLAIN), MySQL stops reading from t2 (for
any particular row in t1) when it finds the first row in t2"
The problem is that DISTINCT+join optimization in not shown in EXPLAIN, one can only
detect it by checking query speed or doing arithmetics on the values of Handler_XXX
counters. The result was that this optimization was once unintentionally disabled:
BUG#32942, and that can happen in the future.
How to repeat:
See testcase for BUG#32942.
Suggested fix:
Make EXPLAIN show that DISTINCT + join optimization are used.
