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FirstMatch strategy and interleaving
>>>>> "Sergey" == Sergey Petrunya <psergey@xxxxxxxxxxxx> writes:
Sergey> I was looking through subquery code and found the following issue
Sergey> with FirstMatch strategy:
Sergey> Our original intent with FirstMatch strategy was to support join orders where
Sergey> sj-inner tables are interleaved with outer tables that are not correlated
Sergey> with the subquery. FirstMatch spec is here,
Sergey> http://forge.mysql.com/worklog/task.php?id=3750, the question of interleaving
Sergey> is covered in section 2.2.
Sergey> [I assumed] I have coded this for non-buffered join execution, both optimizer
Sergey> and executioner support. The first problem I saw was that it didn't seem to
Sergey> be possible to come up with dataset/query that would cause the join optimizer
Sergey> to pick such join order. I don't know whether this is because the cost formulas
Sergey> make the choice impossible or I'm just not finding the right examples. Either
Sergey> way, mysql-test-run suite has no coverage for FirstMatch+interleaving.
Sergey> Now, when I look at the source code and/or force the choice of FirstMatch+
Sergey> interleaving join order by changing costs from gdb, I find out that:
Sergey> - setup_semijoin_dups_elimination() has a bug that will make the query produce
Sergey> incorrect result
Sergey> - Join bufferring now supports FirstMatch with multiple inner tables but
Sergey> doesn't support FirstMatch+interleaving.
Sergey> Since I'm not comfortable with making fixes for something that I can't have
Sergey> testcases for, I'm considering disabling FirstMatch+interleaving. We can get
Sergey> back to it when we have a better understanding of what goes on in the cost
Sergey> Any objections?
I assume it's better to disable something that can't not work than to
keep the old code there (assuming the disabling is safe).
But please put on your TODO to try to create a test case to execute
the suspected code.